Dark matter is “matter” ie stuff in space that we can see (hence the dark bit!). It is undetectable directly, but scientists think it exists because of its gravitational effects on things we can see, like stars. The vast majority of the dark matter in the universe is believed to be nonbaryonic, which means that it contains no atoms and does not interact with ordinary matter via electromagnetic forces.
Its purpose is very hard to determine, especially as we can’t see!
When we had the Big Bang, the universe expanded and still is. Stars, galaxies etc were formed but more is there than we expected. 5 times more material is present in clusters of galaxies than we would expect . Most of the stuff is invisible because the largest structures in the Universe are held together by gravity. So most of the matter in the entire Universe is invisible and is called ‘dark matter’. I guess it’s the bits that we can’t account for. 🙂