Something Science-y I was thinking about this morning

[mjr]

I was watching an episode of "How The Universe Works" this morning, and they were talking about galaxies and dark matter. I started thinking about it, and was wondering if that actually means that there are "dark matter black holes". Or other types of dark matter, like dark planets, dark stars, etc.

[Ironclad Alibi]

All black holes are dark matter. Why do you think they call them "black"?

[Jay 2K Winger]

That's actually not why they're called black holes. They were originally called "dark stars" or "gravitationally collapsed objects," but then an astronomer compared them to the Black Hole of Calcutta, and the name got popular. The name probably also worked because black holes also qualify as "black bodies," objects that absorb all EM radiation.

[Nunavut Pants]

The definition of "dark matter" is "hypothetical matter that does not interact with electromagnetic fields". Black holes do; many of them have insanely-strong magnetic fields associated with them. They are therefore not classified as "dark matter". Current theories are that dark matter, if it does exist, is made up of exotic particles that do not interact in the way that baryons (the regular kind of particles that we are familiar with) do.

It may be possible to have some sort of super accumulation of dark matter, because it should have gravity associated with it. But it is completely unknown if that accumulation would ever get dense enough to have an "event horizon"--that would depend on exactly what kinds of particles make up the dark matter.

But forget about the idea of dark matter galaxies, dark matter planets, dark matter people typing on dark matter computers... Those kinds of things appear to be only in the realm of baryonic matter.

[mjr]

The definition of "dark matter" is "hypothetical matter that does not interact with electromagnetic fields". Black holes do; many of them have insanely-strong magnetic fields associated with them. They are therefore not classified as "dark matter". Current theories are that dark matter, if it does exist, is made up of exotic particles that do not interact in the way that baryons (the regular kind of particles that we are familiar with) do.

It may be possible to have some sort of super accumulation of dark matter, because it should have gravity associated with it. But it is completely unknown if that accumulation would ever get dense enough to have an "event horizon"--that would depend on exactly what kinds of particles make up the dark matter.

But forget about the idea of dark matter galaxies, dark matter planets, dark matter people typing on dark matter computers... Those kinds of things appear to be only in the realm of baryonic matter.

But if dark matter has gravity, wouldn't it theoretically be possible for dark matter black holes to exist? Unless somehow dark energy keeps them from forming somehow.

[Kittish]

But if dark matter has gravity, wouldn't it theoretically be possible for dark matter black holes to exist? Unless somehow dark energy keeps them from forming somehow.

Don't forget about that whole "doesn't interact with electromagnetic fields" part. We don't actually know yet whether gravity falls under that umbrella or not. If it does, then you absolutely would not be able to have dark matter structures like planets or stars or black holes. There's also the consideration that even if you take gravity out of that mix, that the atoms of dark matter might not be able to hook together if they're not subject to electrical or magnetic forces (to drive/pull the atoms together into a condensation cloud).

[mjr]

Don't forget about that whole "doesn't interact with electromagnetic fields" part. We don't actually know yet whether gravity falls under that umbrella or not. If it does, then you absolutely would not be able to have dark matter structures like planets or stars or black holes. There's also the consideration that even if you take gravity out of that mix, that the atoms of dark matter might not be able to hook together if they're not subject to electrical or magnetic forces (to drive/pull the atoms together into a condensation cloud).

Those are great points. But it makes me wonder if there's something like a "dark electromagnetism", that's almost sort of like "dark energy". Because if I'm not mistaken, we know that dark matter has mass (gravitational lensing being one piece of evidence), which tells me it would have gravity, or at least the "dark" equivalent of gravity.

[Nunavut Pants]

But if dark matter has gravity, wouldn't it theoretically be possible for dark matter black holes to exist? Unless somehow dark energy keeps them from forming somehow.

Dark matter has gravity, yes. But we don't know anything about its density. How much mass of dark matter can fit into a given area? Black holes require a lot of mass to be in a relatively small area. It may very well be that dark matter will not let itself be compressed enough to form an event horizon. We don't know enough about it to say.

The main thing is: Dark matter is not the kind of matter that you are used to. Thinking about it as "just like matter" will lead you down paths that are certain to be wrong.