Yes, the effective mass of a photon is determined by E=mc^2 even though it has no mass, but it's sort of by definition because nothing with mass can reach the speed of light.

Also anything moving at the speed of light doesn't perceive the passage of time, because from its perspective we're so time-dilated that the universe comes and goes in an instant as it crosses that expanse. So an argument could be made that only one photon exists, forming a frozen 4D crystal with the shape of every photon's path through the universe.

A black hole's average density decreases by increasing radius. So theoretically a black hole could be formed from a gas like air, and in fact IRAS 20100-4156 with a mass of 3.8 billion suns and a diameter of about 10 billion km (according to the video below) has the same density as air:

https://www.youtube.com/watch?v=71eUes30gwc

Also from the video, if we take the average density of the universe and consider it a uniformly distributed gas, we find that it fits within a black hole 10 times larger than the radius of the visible universe, which is 45 billion light years. So I believe that we're inside of a giant black hole and that the expansion of the universe is driven by Hawking radiation at the event horizon causing it to evaporate, which lowers the radius, which from our perspective looks like galaxies slipping away from us faster than the speed of light as they pass that velocity due to the Hubble constant multiplied by that distance.

So technically a black hole could be created by just photons with a mass energy equivalence similar to air. It would be curious to see what the radius of a black hole filled with the cosmic background radiation would be, and how that correlates to the ~5 hydrogen atoms per cubic meter from the video. It wouldn't surprise me if they're equivalent or correlated so that the background radiation represents how much matter is inside the universe, not just its age since the Big Bang.

In the first 4 minutes of this video, Neil deGrasse Tyson is worried about the end of the universe trillions of years from now, when the last galaxies have slipped outside the observable radius, so that future life will think that the Milky Way galaxy is the entire universe:

https://www.youtube.com/watch?v=rFVdueuDD_o

But I think that the inward rush of the universe's event horizon will continue forever and that even our galaxy won't be spared, even though right now it looks gravitationally bound. Eventually everything will be pulled apart in a big pop as even the force between nucleons won't be enough to resist Hawking radiation within a small enough black hole. Also the universal black hole will catch up to the Sagittarius A* supermassive black hole at the center of our galaxy, pulling even it apart. So eventually everything pops back out into the surrounding universe.

So probably the universe can be thought of as perhaps an infinitely wide nested swiss cheese of black holes. There may be one principle formula that states how that shape comes from quantum mechanics where the long tail of probabilities leads to energy and particles arising (I'm speculating since my studies didn't take me that far), or maybe the physics of the child black holes can vary from their parent universes because they're separated by an event horizon so that our universe evolved physics favorable for life to observe its own existence. Which takes us back to the Anthropic Principle which doesn't really tell us much.

TMI I realize, but hey, it's Sunday.

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Edit: I forgot to add why I was writing this. At large scales, the universe forms bubbles with matter along the surfaces where they meet, and large voids in between. Which suggests that something caused matter to be attracted to those boundaries. I wonder if this is similar to how something like dark matter causes galaxies to rotate faster than they should for how much mass is at their perimeters. It suggests that space is moving there without having an attractor. As if space itself can acquire momentum without mass.

This is sort of how space looks to us as it flows into Earth, causing us to perceive an accelerating reference frame which we feel as gravity. If the ground disappeared and we began falling, then we would no longer perceive gravity or its associated time dilation (except from tidal forces as we approach the center). Which suggests that someone falling into a black hole would remain weightless and stay synchronized with our frame of reference, even as they passed the speed of light from our reference frame and appeared close to frozen to us.

So it might be possible to travel from outer to inner black hole. Although the speed of expansion by Hawking is so large that probably the falling observer would become suspended at the equilibrium point where expansion matches inward acceleration. So in a very real way, the amount of space within a black hole could be much larger than its apparent radius from our frame of reference. Giving some credibility to the idea that child universes exist within black holes.

Also I don't know if there is a gravitational lens effect on photons arriving to us as they cross the heavier space at the edge of galaxies:

A) If there is, then the mass exists there in the form of dark matter, neutrinos, WIMPs, etc.

B) If there isn't, then space appears to be moving without an attractor, meaning that something like a Hubble constant might be missing from our relativity equations or work something like MOND, or that sci fi stuff like warp drives might be possible. I like to think that our universe evolved the conditions for sci fi, and that stuff like dark matter is a wink to us so we can figure out how to do it through something like electrogravitics.