How Do We Know Dark Matter Exists? - Universe Today

How Do We Know Dark Matter Exists?
Dark matter can’t be seen or detected by any of our instruments, so how do we know it really exists?
Imagine the Universe was a pie, and you were going to slice it up into tasty portions corresponding to what proportions are what. The largest portion of the pie, 68% would go to dark energy, that mysterious force accelerating the expansion of the Universe. 27% would go to dark matter, the mysterious matter that surrounds galaxies and only interacts through gravity. A mere 5% of this pie would go to regular normal matter, the stuff that stars, planets, gas, dust, and humans are made out of.
Dark matter has been given this name because it doesn’t seem to interact with regular matter in any way. It doesn’t collide with it, or absorb energy from it. We can’t see it or detect it with any of our instruments. We only know it’s there because we can see the effect of its gravity.
Now, you might be saying, if we don’t know what this thing is, and we can’t detect it. How do we know it’s actually there? Isn’t it probably not there, like dragons? How do we know dark matter actually exists, when we have no idea what it actually is?
Oh, it’s there. In fact, pretty much all we know is that it does exist. Dark matter was first theorized back in the 1930s by Fritz Zwicky to account for the movement of galaxy clusters, but the modern calculations were made by Vera Rubin in the 1960s and 70s. She calculated that galaxies were spinning more quickly than they should. So quickly that they should tear themselves apart like a merry-go-round ejecting children.
Rubin imagined that every galaxy was stuck inside a vast halo of dark matter that supplied the gravity to hold the galaxy together. But there was no way to actually detect this stuff, so astronomers proposed other models. Maybe gravity doesn’t work the way we think it does at vast distances.
But in the last few years, astronomers have gotten better and better at detecting dark matter, purely though the effect of its gravity on the path that light takes as it crosses the Universe. As light travels through a region of dark matter, its path gets distorted by gravity. Instead of taking a straight line, the light is bent back and forth depending on how much dark matter is passes through.
And here’s the amazing part. Astronomers can then map out regions of dark matter in the sky just by looking at the distortions in the light, and then working backwards to figure out how much intervening dark matter would need to be there to cause it.
Large Hadron Collider. Credit: NY Times
These techniques have become so sophisticated that astronomers have discovered unusual situations where galaxies and their dark matter have gotten stripped away from each other. Or dark matter galaxies which don’t have enough gas to form stars. They’re just giant blobs of dark matter. Astronomers even use dark matter as gravitational lenses to study more distant objects. They have no idea what dark matter is, but they can still use it as a telescope.
They’ve never captured a dark matter particle, and haven’t studied them in the lab. One of the Large Hadron Collider’s next tasks will be to try and generate particles that match the characteristics of dark matter as we understand it. Even if the LHC doesn’t actually create dark matter, it will help narrow down the current theories, hopefully helping physicists focus in on the true nature of this mystery.
This is how science works. Someone notices something unusual, and then people propose theories to explain it. The theory that best matches reality is considered correct. We live in a modern world, where so many scientific theories have already been proven for hundreds of years: germs, gravity, evolution, etc. But with dark matter, you’re alive at a time when this is a mystery. And if we’re lucky, we’ll see it solved within our lifetime. Or maybe there’s no dark matter after all, and we’re about to learn something totally new about our Universe. Science, it’s all up to you.
35 Replies to “How Do We Know Dark Matter Exists?”
O yeah – it’s there. Dark Energy? Let’s see that case. All I can find is:
1) The universe is “flat”.
2) There isn’t near enough matter or dark matter to make the universe flat. So there must be something else.
3) the rate of expansion of space between galaxies has been increasing since (I don’t remember) 2 billion after the “big bang”? So somehow whatever is making the universe flat makes it expand?
I wonder if the dimensions hide things somewhat that may express themselves gravitationally (and or beyond gravitationally ala “dark energy”) but not trivially in space-time as we see it.
Take the example of Ptolemy and Hubble- maybe “we” aren’t not just the center of the solar system, or the galaxy, or of all space, but maybe what we see and feel isn’t the center-plate of existence. Maybe we’re just a fringe element.
Actually I wasn’t confusing DM and DE. I’m guessing you misread me or my start was too obscure. I take DM as fairly proven and uncontroversial despite the rest of the comments going in other directions. DE I find much more controversial. I know String Theory isn’t proven – but like I had in the link, it was a subject of previous discussion around here.
Of course we are on the fringe of existence. Think of a bomb going off. Everything is blown out from the center. We are on one of these pieces of the Universe that has been blasted out from the center of the Universe.
What is causing the Universe to expand constantly faster? This is an easy explanation and cause without bringing in exotic terms. First, you need to study explosions. They occur in two parts. The explosion that leaves the epic center in an absolute void. And the collapse of matter back into the center. This happens in all explosions even the big bang.
What we are seeing is not the acceleration of the Universe’s expansion. What we are seeing is the Universe’s collapsing into the center. The strongest pull is being delivered to galaxies nearest the center. Special relativity would make it look from our perspective as just as everybody thinks, the Universe accelerating in it’s expansion. We simply see too small of the section of the pie to see this. We also are seeing too small of a time frame to verify whether this is true or false. To us, everything is drifting away. This is as you would expect. But as galaxies closer to the center decelerate it looks to us as they are accelerating away from us. As our galaxy decelerates, it looks as if galaxies further away from the center are accelerating away from us. Galaxies at the same distance from the center look like they are accelerating from us too as they were launched from the initial big bang explosion at a different angle. And the distances will look like they are expanding.
So relax people. The Universe is behaving exactly as predicted it should react. There is no secret energy out there that is accelerating our expansion. Think of this: Can you see or measure centrifugal force? No. It can be predicted from laws that we already know. So can the expansion/contraction of the Universe.
BlackWolfStanding:
“We simply see too small of the section of the pie to see this. We also are seeing too small of a time frame to verify whether this is true or false.”
No no no, the opposite is true. The fascinating thing with astronomy is that we can observe all times today, all the way back to a tiny percentage of the time since Big Bang. That’s key to the fantastic success of astronomy.
No, no, no. When we look back say 5,000,000,000 years to a galaxy, we see exactly 5,000,000,000 years. We don’t see the exact same galaxy at 5,100,000,000 years. Now when we look back closer to the big bang say 13,000,000,000 years at a galaxy. We see that galaxy at 13,000,000,000 years. We don’t see that galaxy at 13,100,000,000 years. In other words, we don’t see movement or enough movement to make certifiable predictions of what is actually happening.
And of course the Big Bang was an explosion. It was on such a massive scale that our minds just can’t comprehend what happened. But if it walks like a duck and talks like a duck, it must be a duck. When all the energy in the universe came into existence from one sub-plank length object (very small indeed) at that time everything behaved exactly as an explosion should behave. The shock wave expanded outwards setting the boundary of the known Universe. The force of that blast was so huge that it propelled the shock wave faster than the speed of light. As the energy of the original blast diminished, so did the velocity of the shock wave. After billions of years, the edge of the Universe (the shock wave) is finally coming to a halt. It’s not stopping because of friction. It’s stopping because the Universe behind it is stopping and the leading edge is being draw back into the Universe. What’s outside of the Universe (beyond the shock wave) is anybody’s guess. Could be an absolute void with no radiation at all. Could be some type of plasma. Could be almost anything. But what is certain is the center of the explosion is an absolute void which must be filled. And the galaxies closest to that void are going to fill it first. The galaxies at the farthest edges of the explosion might have reached escape velocity and will never come back. And we are somewhere in the middle without knowing what our fate will be. Are we close enough to the big bang’s center to be drawn back into it? Did we reach escape velocity? Or are we going into orbit around some yet undiscovered grand father of a black hole that is holding the Universe together?
Because of special relativity any number of outcomes could be possible and we will never truly know it.
— “And of course the Big Bang was an explosion. It was on such a massive scale that our minds just can’t comprehend what happened. But if it walks like a duck and talks like a duck, it must be a duck.”
That is the mindset of pseudo science. You are trying to shoehorn your theories using philosophical debate: Because is has the word “bang”, then it must be an explosion.
The thing is, your pseudo theory breaks down the moment you try to put it into maths and number. Your so called shock wave. How big is it? Give some estimation in number. How massive is it, give some estimation in numbers. Give some maths, to calculate your hypothesis instead of only using philosophical reasoning.
— “No, no, no. When we look back say 5,000,000,000 years to a galaxy, we see exactly 5,000,000,000 years. We don’t see the exact same galaxy at 5,100,000,000 years.”
Of course not we are in linear time. You see a snapshot if that light that departed from there 5 billion years ago. But when you look between the void in that galaxy you look back 13.8 billion years. That galaxy blocks your view of objects behind it. When you look at the moon now can you see the moon from 2 weeks ago?
And there are other other galaxies just like that one but found 5.1 billion years away right next to it. So we do have a clue what they looked like.
Why no math to accompany this? This isn’t college and this is a very poor median to display equations on. Just simple idea’s will take pages of complex proofs.
But let’s examine what happens in a explosion. After ignition, everything is very hot. Right after what ever caused the big bang, everything was very hot. As an explosion expands, everything cools off. After the big bang, everything cools off. At the center of an explosion, there is a void. Material from the explosion fills back into the epic center.
Now, let define what an explosion is. The sudden and uncontrolled release of energy whereas all the energy is released virtually instantly. Bombs are a good source for explosions, but are not in themselves the explosion. A star going hyper-nova is a good source of an explosion, but it is not the explosion. The big bang had an uncontrolled release of energy. This was the explosion. Whatever contained the energy prior to the big bang, we don’t know. And we will never know.
Now when we study the comic microwave background, that is the ‘shockwave’ that I was talking about. The center of the Universe will be a hole. It might actually be very hard to find as dark matter, black holes, galaxies, etc… will be obstructing our view. But that brings me to the cosmic web. When studying it’s structures, there are slight arcs in it. Those arcs could be extended to show us the true size of the Universe. But before extending them, probably want to move all the galaxies to where they would be now, not 5,000,000,000 or so years ago.
But like I said, we don’t see enough of the pie and as the universe continues to expand, we will be seeing less of the pie, not more.
Everything I just said doesn’t change any current theories. Just sums up what everything is leading up to.
Ok, here goes a completely moronic rant for your amusement. A) I don’t believe in DM. I believe scientists just don’t like being wrong. That being said, B) We seem very concerned about breaking the light-speed limit. We are as close to that as we are to human teleportation. Yet there is a tool in my kitchen (as well as in my took box) that can vary in temperature more than what separates us from another (and greatly overlooked IMHO) “limit”. That’s right, my oven can reach over 300 C, more than separates room temperature from AZ (not Arizona, but 0 K). Ok, so what does that have to do with DM, DE or anything else? Well, look at the boomerang nebulae. Coldest [natural] place we know of. Why? Expanding gas cooling (being simple here, I know). So, imagine if there was some huge amount of matter that was too “hot” to combine, and then, it all of the sudden underwent some hyper-rapid expansion, maybe through some sort of large boom, or big bang even. This expansion could have had enough power to push things past the AZ limit, far beyond in fact. So, as an example, let’s take water for a comparison. Let’s take the melting point to be AZ. Now, let’s take a piece of ice at -2 C and another at -80 C. Both same state, but one would require more energy to get to phase change. Now, unlike HOH, what do we know about any particle dropped below AZ? Nothing really, but would a particle like that even interact with any normal particles? Yet, they would still be matter, so they would have an effect on the space/time fabric (gravity). Now, as energy is transferred, these particles pass the “magic point”, they appear to us and interact like particles that we are used to observing. This also releases some energy (as most transitions of phases do). Which brings us to, well, you know what. But as I said in the beginning, it’s probably a SWAG or fudge-factor that someone needs to make their equation work. IMHO, of course.
It is a wall of text, and so far nothing makes sense in your wall of text.
And the reason that nothing makes sense in your wall of text is because you use logical reasoning with flawed ideas.
This is why ideas must be tested in reality. Sounding logic is not enough. You must be able to measure it in reality and also include experiments that would disprove your hypothesis.
Now in order to measure in reality you have to convert your ideas into maths. And you have other people to check your maths and repeat the experiments under different conditions and with different experiments.
What isn’t discussed here is the Electric Theory of the Universe. Dark Matter is theoretical. ‘To call “dark matter” and “dark energy” DISCOVERIES is a violation of a long-cherished scientific principle, one calling for the rigorous separation of undisputed observation and theoretical conjecture.’
[“Thunderbolts” promotional video URL link deleted by moderator.]
“What isn’t discussed here is the Electric Theory of the Universe. ” … because it’s wrong and really, really stupid. And in science, “Theory” isn’t the same in meaning as one guy in a bar saying, “I have a theory about why blondes are stupid”. Scientific theories hold vast amounts of weight and in fact in many cases are as rigorous as laws. Quantum theory for instance is a proven fact because solid state electronics like diodes and capacitors existence are all based on it. The “Darks” have lots of evidence supporting their existence. All science is foggy until the last of the mist is blown away by observations and experimentations.
I’ve been reading Carl Sagan’s “The Demon-Haunted World”, which is a comparison of the scientific method of understanding the world, versus pseudo-science (astrology etc) and religion. Sagan makes the point that it’s just about always easier to be kind to yourself and pretend that your hypotheses are OK, than to be scientific and be your own critic. He quotes Benjamin Franklin (the face on the Dollar Bill) who ruefully remarks on how many theories he came up with, only to slide them in the dustbin when they did not measure up using the scientific method to test them.
Sagan’s book comes across as somewhat pessimistic about America and the education system in America.
After reading these comments (and Olaf’s attempts to get people thinking as self-critical scientists rather than people who believe without judgement or evidence), I am beginning to think that Sagan’s pessimism has some basis in fact. Americans seem to be choosing to believe salesmen, and engaging in no critical enquiry about their sales pitch … yet I see Americans as the very people who have materially benefitted most in all the world from the scientific method.
Perhaps in line with this, Fraser might wish to consider a short post on science education and How It All Works.
Regards,
Tony Barry
Thank you, Tonybarry!
And I especially like your suggestion that Fraser should do a bit on American education *along the lines of Sagan’s critique*.
Pseudo science has taken ahold to a terrifying degree in our press and even in some places that presume to teach the real thing.
Thanks guys. I live in Australia and right now we are going through a phase where the government appears to value science less and less, putting less funding into research, removing the Climate Change Commissioner, putting Science as a part of the Department of Industry rather than having a Minister in its own right …
This focus is not in our best interests as a country.
The spinoff is that more people see it as OK to engage in pseudo-science, to consider the results of pseudo-science as equal to genuine peer-review, and to get upset when reality does not conform to their odd notions. Sometimes it starts to scare me just how many people are willing to believe in things without any evidence. And they spend their money on things that don’t work. This is not because they are stubborn – it is because they really do not know how to assess truth versus falsity.
Errr, that would be George Washington’s face on the dollar bill, Ben Franklin is on the $100 dollar bill. Nonetheless, Sagan’s book sounds interesting, I’ll pick it up if it is available as a Kindle download.
Hi William,
Thank you for the clarification. I have not actually seen American Money much at all. Here in Aus we have different notes, which are worth less than yours. Good for American tourists to Australia, not so good for us going there. I was reading Robert Heinlein, who mentioned the “Benjamin Franklin” as the face on an (unspecified) piece of American currency, and incorrectly assumed this was the unit dollar bill.
Yes Sagan’s book is available as a Kindle download. That’s where I read it on. I’ve been doing a lot of reading of science stuff on the Kindle of late.
Regards,
Tony Barry
Dark Matter = Luminiferous Ether theory 2.0. It’s an embarrassment that it’s even discussed as a plausible possibility when physicists don’t even understand what the nature of gravity is, let alone can’t connect it to the other forces. The only concern we should have with dark matter is when it clogs the toilet, which is where this theory will end up.
DM is a natural human attempt to explain the unknown. It’s merely a suggestion at this stage, but yes, with gravity still posing questions, DM really is far fetched, although the name is reassuringly vague enough to appease the scientific skeptic.
Hi Dave,
Dark matter is advanced as a solution to the observed problem that galaxies rotate too quickly for gravity to hold them together.
The competing solution is Modified Newtonian Dynamics (MOND) which has fewer adherents.
Both theories offer solutions to the problem. Neither theory is accepted as fact. In scientific thinking, theories require a lot of corroborating evidence to get anywhere.
There is however some grace supplied to the discussion. People have time to consider the evidence, and to discuss it, and to offer modifications to the theory to account for the evidence. It’s a lot of discussion to get to the point of being accepted as a reasonable answer.
Your postulation that Dark Matter = Luminiferous Ether and therefore untenable is … interesting … but seems to be short-circuiting the necessary discussion, and the postulation does not offer a means whereby the premise could be tested.
Without arguing the correctness or not of dark matter, the truth is that dark matter is an unscientific concept. I mean this in a Popperian sense, that proposing the existence of something that cannot be seen or detected is by very definition unfalsifiable, and therefore outside of the criterion of demarcation of normal science. It is therefore a pseudoscientific theory, even if it is true.
Also the statement that “We live in a modern world, where so many scientific theories have already been proven for hundreds of years: germs, gravity, evolution, etc.” — isn’t 100% true, since the 1930s we just don’t say things are “proven” any more, at best we can say that the observations and data strongly support a hypothesis.
What is dark matter? The article offers a good possibility: “Or maybe there’s no dark matter after all, and we’re about to learn something totally new about our Universe.”
The existence of dark matter is inferred mostly from the characteristics of galactic rotation: “most stars in spiral galaxies orbit at roughly the same speed. . . . These results suggest that either Newtonian gravity does not apply universally or that, conservatively, upwards of 50% of the mass of galaxies was contained in the relatively dark galactic halo. (source: http://en.wikipedia.org/wiki/Dark_matter#Galactic_rotation_curves )
No dark matter is needed, just an alternative conclusion: Stars in a galaxy do not “orbit” the central bulge. Their motion is NOT comparable to planetary orbits in a solar systems. This is a much different situation.
Picture two small galaxies approaching each other. The chances are good that the approach will be off-center (not co-linear). Two effects will become apparent immediately. The differential effects of gravitation will cause the galactic blobs to “string out” into a line of stars. The off-center approach will cause the system to rotate around its barycenter (forming a spiral). The barycenter core is intially formed from the stars on the leading edge of each galaxy which experience a stronger gravitational pull and form a central nucleus of stars, usually accompanied by a visible “bar” of stars connecting the leading edges of the strung-out stars. Graviation changes the DIRECTION of the stellar motion far more than the speed. The result is that, as quoted above, “most stars in spiral galaxies orbit at roughly the same speed”. And they do so because their original SPEED of approach remains mostly unchanged.
It is not rotation or “centrifugal force” that keeps the stars separated. The separation is maintained by the same mechanism as with non-rotating star structures like globular clusters. There is a mass-dependent distance where gravitation and the outward expansion of space are at an equilibrium. Gravitation has an inverse square force distribution, but the expansion of space is centerless and uniform. There is necessarily an equilibrium position for these forces. For stars it is a few light years; for galaxies, it is a few million light years. Hence, the galactic stars will not coalesce with each other, but they are still stuck inside the galaxy.
You and other readers may like to search and research The Theory of “Dynamic Equillibrium”. The same titled theory independently by both Einstein amd Lemaitre. It went into Einstein’s desk drawer unpublished in 1931. Lemaitre published the theory with nearly identicle features in the 1920s (1927 from memory). It is an interesting theory incorporating the big bang’s expansion and at the same time predicting and conforming to many modern mesurements of the age of the universe, hubble constant and other factors.. Interested in any reactions or opinions readers would care to make.
Thanks for the insights. I am not an astronomer, and so that kind of information is not at my fingertips.
My quick search turned up this little snippet from “Universe boundary in Einstein 1931 same as Lemaître 1927” (http://adsabs.harvard.edu/abs/2015AAS…22521504S) which says: “. . . universe in balance, changing but always steady, eternal but ever-reborn, is exactly what we observe.”)
The question of “how big is the universe?” has always bothered me. That, and the question of, Does it continue to expand, or will gravitation cause it to collapse? This is made worse by the fact that the Universe has both “local” and “non-local” behaviors.
If someone hands me a closed cardboard box, I can then define a “size” for the box, and also what I mean by an “inside” and an “outside”. But I must have the physical box first; the box is not “made of” an inside, an outside, and a size. It is made of cardboard. . . Modern astronomy has been concerned with the inside, outside, and size of the Universe, but has not defined “the box” itself. Additionally, the thinking now seems to be that the Universe is both centerless and edgeless (fundamentally “non-local”). Maybe we have been asking the wrong questions.
This is related to the problem of defining space and time. Time is usually parametric in our equations (even in quantum mechanics), and so physicists are increasingly regarding it as non-fundamental (as Einstein did). The view gaining currency now is that “motion” is more fundamental and would make a better metric. In other words, let “c” (the speed of light) BE the “box” and let space and time be defined by that box. The space and time units are always changing (progressing) but the RATIO remains constant. In the words of the above citation it is “in balance, changing but always steady, eternal but ever-reborn.” The progression of space and time (“space/time” instead of “space-time”) would become the new Ether–the cardboard of the box, viz., something physical, though immaterial. It is non-directional (“non-local”) and cannot be detected by a vector dependent experiment like that of Michelson-Morley. (There is more about non-local physics in my comments at: http://www.universetoday.com/108044/why-einstein-will-never-be-wrong/ http://www.universetoday.com/78558/albert-einsteins-inventions/#more-78558 )
Gravitation should divide itself into three regions: 1/d^2 (ordinary gravitation); 1/d^1 (Hubble region gravitation) and 1/d^0 (no gravitation at very extreme distances). The speed of light (really the speed of the Ether as a limit) should be constant, but we MEASURE it from a gravitational reference system, and it should show small variations. Likewise, the Hubble constant will not truly be constant, but would be dependent on what kind of gravitational reference (galaxy) is used. At extreme distances there is no gravitation to oppose the expansion, and “ordinary” galaxies should have a redshift of z=1. (However, half of that measured redshift “physically” belongs to the Milky Way, as it participates in the expansion also.)
The current, rather circular, argument that we don’t really understand or are really able to explain what it is that we are actually seeing when we observe the apparent behaviour of galactic masses across the universe – therefore we create ‘Dark Matter’ and ‘Dark Energy’ as a tool to fill in the gaps in the knowledge and to make the observed universe actually work in a manner we understand – then go on to say that we know it exists because without it then the universe wouldn’t work in the way we understand is perhaps, in itself a key to understanding the core problem.
To extend that logic to the point of accepting that it is not ‘The Dark Forces’ that we are failing to comprehend but the full, interactive spectrum and all its convoluted behavioural structure that we have not yet fully got to grips with.
It is far from uncommon to come up with an explanation for an observed phenomenon, to spend significant resources on attempting to prove that hypothesis, only to find that it was a blind alley all along. That is the normal function of science at its best !
We know, and can observe, the effects of and the interaction between gravity and energy propagating through gravitational fields but we are far from a full understanding of all of those effects and of those interactions.
To observe a phenomenon, to propose a theory to explain it, usually functions on a basis that ‘it looked good at the time’ but there has rarely been a time when it has provided the final answer. Science is has a long history of well-packed ‘recycle bins’
We may indeed now be on the cusp of a significant re-think of the whole gravity/energy interactive structure that may give a whole new view of the universe and its origins. What we are in need of most of all is someone ‘out in left field’ to turn on the light !
The answer to this question is easy: We don’t know it exists at all.
Dark matter was proposed by the “mathemagicians” because gravity was too weak to account for spiral galaxy formation. It is a theoretical concept that was constructed to salvage the failings of the gravity based theories of cosmology. These same mathemagicians have also given us black holes, accretion discs and neutron stars. None of these theories are based on observation and they defy confirmation. These concepts also require the known laws of physics and chemistry to break down.
Using one unproven theory as the basis for the next unproven theory is hardly consistent with the scientific method.
Cosmology today resembles more of a religion than a science. All it takes to adhere to the standard model is a firm belief in the impossible.
Cosmology
It’s called “successive approximation” in other fields. We make an estimate, refine it, get closer to the reality. Rinse, repeat.
Nobody expects it to be absolutely correct. But it is a little better than saying that galactic rotation is accomplished by pixie dust, the hand of the Flying Spaghetti Monster, or other non-rational agents.
Dark matter is the simplest rational explanation for the observations of galactic rotation. Not necessarily the correct answer. But the advantage of simplicity, in the absence of further info, is a compelling advantage that more complex answers must overcome to be considered.