How do you explain dark matter to kids?
I tell them that when they touch the table, what they are feeling is the electromagnetic repulsion (or, you know, I use some less jargony way of saying that) between that surface and their hand, and that is what makes it feel solid and what keeps you from passing right through it. Dark matter doesn’t seem to have that force. It has gravity, but it doesn’t do electromagnetic repulsion as far as we can tell. So if dark matter were in the room (which it probably is), it would pass through you unnoticed. And we know dark matter is out there because of the way it moves things around in galaxies and clusters of galaxies, and how its gravity bends light.
Do we have evidence for dark matter?
The evidence that dark matter is a real component of the Universe – not an alteration of gravity – is pretty overwhelming at this point. We see evidence for dark matter in so many places now (motions of galaxies, motions within galaxies, hot gas in clusters, strong and weak gravitational lensing, gravitational microlensing, cosmic microwave background anisotropies, the matter distribution on large scales, and the collisions of galaxy clusters, for instance), it is really impossible to ignore. Attempts to just change how gravity works to fit the data don’t work when you take all of these observations together. There are places on small scales (like with dwarf galaxies) where there seem to be difficulties bringing the simplest dark matter theories into line with what we see in observations, but even those are not always compelling anomalies.
What progress has been made identifying dark matter?
Short answer: it is all a bit of a mess at the moment. There are a few dark-matter-detection experiments giving us different answers, so it is really hard to say. But there are a few things we are pretty sure we do know about dark matter: it is fairly cold (non-relativistic in its motions), it is probably some kind of fundamental particle (though there are certain models of very low-mass primordial black holes that aren’t yet ruled out), and it doesn’t seem to have significant non-gravitational interactions (in the sense that its only major, easily detectable, interactions with itself or anything else are via gravity).
There have been lots of really interesting hints lately of possible signals of dark matter’s particle physics effects in astrophysical observations, but it will probably be a few years yet before it all gets sorted out.