Dark matter is invisible, but its gravity governs the motion of galaxies. What it's made of is a mystery. One idea: it consists of ultra-light particles akin to axions.
Dark matter is the invisible scaffolding of the universe. It's there, we know from the motion of stars and galaxies, but what this scaffolding is made of remains unknown. One guess is ultra-light particles similar to axions. They barely interact with ordinary matter, yet occasionally they do get involved.