The real universe is happily chugging along with the scientists of the real universe trying to figure out why their calculations predict about 20% more matter ought to exist than what actually does. Little do they know, we’re that 20%, happily living our lives while occupying the exact same space at the exact same time as they are, each of us oblivious to the other because our particles don’t ever bump into theirs.
I don’t belong in that thread. I’d be the guy eagerly raising his hand and saying “ooh, pick me!” while everyone else in the class is saying “shut up dude, we all just want to go home!”
From the little bit I could understand of that article, that seems like exactly what I’m thinking of.
So, let’s say this stuff exists, and it’s all over the place. It’s forming stars and planets and galaxies that we’ll never see, except we still experience the gravity from it. Some of it will even form black holes, but since gravitational effects are pretty much the only way we can detect black holes in the first place, we’d never know if a particular black hole is made of dark matter, or “real” matter! In fact, two black holes, one “dark” and one “real” could end up taking up the same space, and multiplying the gravitational effects of each other.
My understanding is that in general, evidence indicates that dark matter does not interact with itself in the way that matter does. When galaxies collide, there are gravitational interactions due to mass. However, particles that collide also interact, adding an additional component to the interaction (glowing gas, changes in energy and direction).
Evidence from galactic collisions seems to show (we can only infer dark matter behaviour) that the dark matter halos interact in a purely gravitational sense and that other interactions are minimal. The halos appear to move through each other while the non-dark mass slows down and heats up. The still moving dark matter halos are revealed by the gas they do drag with them. This would tend to support the WIMP (Weakly Interacting Massive Particle) hypothesis, and counters the MACHO theory (in such a collision, compact massive objects would interact and heat up, revealing themselves).
However, we have a long way to go before we can be sure about anything relating to dark matter.