The idea of dark matter is not for the faint-hearted - so if you are easily confused, you may want to look away now. Essentially, astronomers have looked at the motions of some galaxies, and calculated that the mass needed to gravitationally hold them together is far more than the mass that can be attributed to the stars we see in the galaxy.
In other words, there must be a significant amount of unseen mass or dark matter. In the case of some spiral galaxies, it's been calculated that around 90% of the galaxy must be composed of dark matter.
The weird thing is that scientists still do not know what dark matter really is. Some scientists put it down to huge numbers of long-dead stars, unseen planets and black holes, whilst others have suggested the existence of strange, but so-far-unseen, particles that roam the universe. On the other hand, it could be that the laws of Physics that we have come to rely on break down when considering long distance scales.
Whatever the reason, it is clear that a great deal of research is needed to investigate why are measurements of mass due to light and mass due to gravity do not add up. Something odd is going on - and we need your help to find out what.