OT: Lost in the Galaxy? No wonder.

[douglas]

Kamog said:
OK, this might be a dumb question, but...

Why do astronomers have to introduce the idea of this mysterious "dark matter" to account for the gravity in the galaxies? Couldn't the extra mass simply be made up of planets, asteroids, dust, black holes and other objects that don't emit light and are therefore hard to detect?

All those planets, asteroids, dust, black holes, etc. would by definition be dark matter. Dark matter isn't some wierd kind of exotic material, it's a general category for a whole lot of mass that astronomers calculate has to be there to account for certain gravitational effects but that is hard enough to detect that they haven't found it yet.