<human>:  Can you help me understand the concept of dark matter?
<bot>: Dark matter is a mysterious form of matter thought to make up most of the matter in the universe. It is invisible, meaning we can't see it directly, but can only detect its effects on other matter, like stars and galaxies. It has gravity, but is otherwise difficult to detect. Scientists believe it exists because of the way galaxies and other large structures move and interact in the universe.