Most of the things we see out in space are objects that emit light, such as stars or glowing gases. When we see other galaxies, we are seeing the glowing stars or gases in that galaxy. However, scientists think that matter that emits light only makes up a small part of the matter in the universe. The rest of the matter is called dark matter.

Because dark matter doesn’t emit light, we can’t observe it directly. However, we know it is there because its gravity affects the motion of objects around it. For example, when astronomers measure how spiral galaxies rotate, they find that the outside edges of a galaxy rotate at the same speed as parts closer to the center. This can only be explained if there is a lot of extra matter in a galaxy that we cannot see.

So what is dark matter? Actually, we don’t really know. One possibility is that it could just be ordinary matter—protons, neutrons, and electrons, like what makes up the Earth and all the matter around us. The universe could contain lots of objects that don’t have enough mass to glow on their own, such as large planets and brown dwarfs, objects larger than Jupiter but smaller than the smallest stars. Or, there could be large numbers of undetected black holes.

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