The idea of dark matter is hard - there is stuff in space which we know is there but cannot see! We see the effect of dark matter on the objects around it. We know it must have mass, and therefore gravity. One example is by looking at how spiral galaxies spin. It has been found that the mass needed to hold galaxies together is far more than the mass that we can see.
There must be a large amount of unseen mass, or dark matter. In some cases around 90% of the galaxy must be made of this dark matter.
Scientists still do not know what dark matter really is. Some think it's made of long-dead stars, unseen planets and black holes. But these are made of particles we can usually detect. Others suggest that there are new, strange particles, not yet seen. It could even be that the laws of physics which we rely on, break down at long distance scales.
There is still a lot of research in this area to see why the measurements of mass due to light and due to gravity do not add up.