What is dark matter?
This is not the place for a full discussion of this topic but if you are convinced that at least 50% of the mass of our galaxy is invisible, we have a bit of a problem - after all, our galaxy is nothing special and what is true of our galaxy is probably true of the universe as a whole.
Possible candidates for dark matter include:
- Brown dwarfs (stars < 0.05 MO which never lit up)
- Cometary type matter (rock and ice)
- Dust
- Gas
and lastly, and controversially
- WIMPs (Weakly interacting massive particles)
The trouble is that evidence for any of these is distinctly lacking.