Mark Megahan
Dark matter has been driving physicists insane for decades. It has to be there but nobody can find it. Nobody has more than a few clues as to what it could be made from. Something a lot more baffling, is that there are galaxies out there which don’t seem to have any of the stuff.
Dark matter dilemma
Paradoxically, patches of missing dark matter help prove that it really does exist in the first place. The experts only know what they do about the strange and mysterious stuff from the gravitational effects it has on the universe around it.
What we do know is that it’s everywhere, it has mass, and there simply has to be five times more of it than normal matter. On top of that, “it’s the primary driver of the formation of cosmic structures on all scales.”
The things we image with the fancy telescopes are all “large-scale” structures like “big spiral galaxies, giant ellipticals, groups of galaxies, and rich clusters of galaxies.”
The way gravity holds it all together, they know “dark matter dominates what’s out there.” While the predictions say that it’s everywhere in the universe in general, it isn’t everywhere in particular.
There aren’t many of the dark matter free galaxies around but they seem to have something in common, they’re small. A few of the tiniest ones have almost none of the substance.
At first glance, it looks like a flaw but astrophysicists start grinning about it. It’s “actually a feature,” they say. It narrows down the possibilities. “There are only two plausible ways to create a galaxy without dark matter in a universe that’s rife with it.”
Simulating the universe
Now that we have supercomputers which can model the whole universe, the number crunchers found when they ran the first model through that “simulation to truly test the plausibility of both mechanisms has shown that one of them, in fact, actually reproduces what we see with extreme accuracy.”
They call that a “triumph for the theory of dark matter.”
In that crucial split instant when our universe inflated from a single point of nothingness, “the initial fluctuations that were imprinted on our observable universe during inflation may only come into play at the ~0.003% level.”
Even so, “those tiny imperfections lead to the temperature and density fluctuations that appear in the cosmic microwave background and that seed the large-scale structure that exists today.” Dark matter came later.
Our universe “as best as we can tell, was born with 99.997% perfect uniformity, with underdense and overdense regions that deviated by only ~0.003% from the average: about 1-part-in-30,000. Still, that’s plenty, and after a few hundred million years, stars and galaxies began to form, growing from these initial seeds of structure.”
“Matter, both normal and dark, gets attracted to the densest nearby regions, creating a filamentary structure of mass known as the cosmic web, separated by enormous voids.” Galaxies then form along the filaments, “while the places where the filaments intersect give rise to larger structures like galaxy groups, clusters, and even superclusters.“
