One of the challenges confronting astrophysicists is figuring out how galaxies form. The problem arises in kind of a round-about way.
The space the fills our universe is remarkably uniform. That’s surprising, because it formed from an extremely violent context. We would expect it to be warped, in the mode of Einstein’s general relativity, causing light to “bend” as it traveled the great distances between galaxies. In addition, until a couple of years ago it was believed that the universe was coasting to a stop. In other words, the mass of the universe appeared to be just enough to keep the galaxies from flying apart forever, but not so much that they would turn around and collide together in a “big crunch.”
These two questions were reconciled with Alan Guth’s “inflationary universe” hypothesis. This holds that the universe was created with an invisible, uniform background energy that dissipated very early, creating most of the matter that we see around us.
One consequence of this model is that matter should be distributed uniformly in the universe. This is a problem for galaxy formation, because if matter is distributed uniformly, there’s no reason for it to start clumping together. There have to be little pockets of higher density for galaxies to form. When only normal matter is included in the simulations of the early universe, galaxies form way to slowly, and don’t exhibit the large-scale structures that we observe in the deep sky surveys.
Worse, when we look around the universe, we can’t actually see enough visible matter to account for the gravitational braking that slows down the rushing apart of the galaxies.
One way of solving these conundrums is “dark matter.” The proposed properties of dark matter are that it does not emit light (it’s dark) and that it has a different kind of mass that causes it to clump together to seed the formation of galaxies.
Today we have a negative result from an experiment designed to detect dark matter. This won’t deter the theorists for long – they’ll just come up with new forms of dark matter that are invisible to the detector (this is an old trick, which caught out my thesis adviser back in the ’80s). But it does seem to make Occam’s razor cut more in the direction of the generative orders proposal for the formation of the early universe. That model doesn’t need inflation or dark matter or a multiverse to work. It anticipates just the universe that we see around us.
*sigh* Just saying.