Yes, energy has mass through E=mc^2, but information also has mass:

https://bigthink.com/hard-science/dark-matter-theory/

https://physics.stackexchange.com/questions/583906/does-info...

Probably what's going on is that spacetime contains information that scientists haven't properly accounted for yet. That information could also be a lot more massive than expected. It's a little like how the Casimir effect is so powerful on microscopic (quantum) scales that it leads to large estimates of the zero point field energy and the Higgs field being 246 GeV/c^2.

https://en.wikipedia.org/wiki/Zero-point_energy#The_Higgs_fi...

https://en.wikipedia.org/wiki/Spontaneous_symmetry_breaking#...

So for problems like galaxies spinning more rapidly than expected, the traditional way of handling that is to say that "dark matter" adds missing mass that we can't see, which keeps the stars from flying apart.

But there are other ways of thinking about it. Space itself may be moving as it spirals down the drain of the black hole at the center of the galaxy, like with frame dragging, so stars perceive their local velocity as smaller than it is. So we don't perceive it within our solar system, but we can detect that we're moving too fast through our galaxy. We'd probably be able to detect the redshift from that though, so I'm probably making this one up.

Or space may be flowing into our reality from a higher dimension, lengthening the distance between matter over time, which we perceive as dark energy. Which could be due to the multiverse collapsing into our local reality. In other words, superposition may contain more information than the collapsed state, so the evolution of our reality gradually releases that bound up potential as dark energy. Admittedly, this is such a fringe idea that I'm probably also making it up.

My favorite interpretation is that the universe is inside the event horizon of a giant black hole. As a thought experiment, imagine being a 5th dimensional being who could observe what happens within a star as it collapses into a black hole. At first, the star is propped up by fusion energy keeping its gas hot enough to stay spread out. As the nuclear fuel runs out, the gas gets cooler and dense enough to begin forming a black hole at the center. At first, the black hole is small enough that Hawking radiation causes the black hole to explode faster than matter can fall in. So microscopic, short lived black holes form and die at the center constantly. We may even perceive matter falling in and coming out changed as fusion, but I digress. Hawking radiation rate is inversely proportional to mass, so eventually enough holes form that all of the matter falls in before it has time to radiate as energy.

So far, so good (I haven't said anything outside of current physics). But think about what the matter sees. As it falls into microscopic black holes and pops out again as energy, it's as if there is no floor beneath it. It's like it fell into a little bottomless pit, started accelerating faster and faster into the outward z axis of the hole, then saw the event horizon around it begin to shrink as its micro-universe evaporated, right before it popped out again as Hawking radiation. In other words, the matter perceives space around it growing for a time, before shrinking again. We can think of the interior of the hole as adding space as the floor falls at near the speed of light, even though we see a fixed radius event horizon from outside.

This is exactly what we perceive in our universe with expanding space and the Hubble constant, with distant galaxies leaving us as they exceed the speed of light at the radius of the universe 14 billion lightyears away and leave our reality:

https://www.quora.com/I-calculate-the-light-speed-divided-by...

I think this is probably why the James Webb telescope is seeing more galaxies at high distance (which shouldn't have had time to form) because the universe is actually continuous and looks like a big block of swiss cheese with enormous black holes bumping up against each other. There might never have been a Big Bang, instead, inflation was space growing inside of the newly created black hole that our universe is inside of. Time may even reverse within the horizon, which has implications for things like antimatter and the weak interaction breaking parity symmetry (of CPT).

So we would see smaller black holes in our universe gaining mass at the same rate as space is expanding, as if dark energy is pouring into them, which sounds like the best description so far of how non-primordial black holes could reach a billion solar masses. Which has all kinds of ramifications for quantum gravity and the manipulation of mass.

I'm not saying that any of these models exactly describe reality, just that these are breadcrumbs you can use to think outside of current dogma within the physics community. Also I'm not a physicist, just a programmer searching for abstractions that might be more accessible than the gobbledygook which creates a kind of passive gatekeeping around these concepts.