by Hossam Aly: What is dark matter?
Yes, earth is surrounded by dark matter particles, though they’re probably a bit sparse in the solar neighborhood. Dark matter particles are moving through your body, and the whole earth, unnoticed as we speak.
Here’s what we know about dark matter:
We know there must be a lot more dark matter than Baryonic (normal) matter. We know that galaxies are embedded in fuzzy dark matter halos. We know that galaxies wouldn’t exist, at least in their present form, without dark matter halos that keep them together (which was historically the first evidence, though perhaps not the most convincing, for dark matter). We know that galaxy clusters, which are ubiquitous in the universe, wouldn’t be possible without dark matter. Galaxies would instead just fly by each other without being able to get gravitationally bound and form groups and clusters.
We are able to deduce the dark matter density profile from rotation curves for many galaxies, including our own. In a few cases, we’re able to map dark matter halos using gravitational lensing (check out), which I think is the best evidence we currently have for dark matter.
We know dark matter decoupled from radiation a lot earlier than Baryonic matter did (the latter happened at the moment of last scattering, when the universe became transparent for the first time 380,000 years after the Big Bang). This allowed it to collapse and form structures and over-densities that were the bed for galaxies to form afterwards. If you’re into cosmological simulations you’d know it’s impossible to form galaxies that early in the history of the universe with feasible properties without dark matter. We know that dark matter forms a sort ofof filaments and sheets that intersect at nodes where galaxy clusters form. We know that it’s impossible to reproduce the observed acoustic peaks in without dark matter.
Here’s what we don’t know about dark matter:
What it actually is.