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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? 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?
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The 'why' is that the universe is not behaving right for the observed material. It looks like there is more gravity than the currently known contents of the universe can account for. So, there are various ways to invent more mass -- either simply 'dark' but normal matter as already mentioned, or also as 'exotic' stuff that doesn't even interact with normal matter except through gravity.
It should be pointed out, though, that we might not have mastered how gravity
actually works yet. There is a very detectable discrepancy in the movement of the Pioneer space probes (currently the furthest man-made objects from earth). They have not moved as far as they should have. Not by much, but by enough to make the NASA engineers and scientists wonder what is going on. And even though they are moving in opposite directions (i.e. on opposite sides of the solar system), they show the same degree of this discrepancy, too, so it's not easily explained by some hidden planet somewhere. There may be an extra 'fudge factor' in the way gravity works over great distances that would explain the movement of galaxies without requiring all that 'dark matter'.