[Kamog]

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? Do all planets have to orbit stars? Couldn't there be just lots of big planets in between the stars, independently orbiting the galactic center, so that they make up the missing mass?