It is thought to reacted less with normal matter because we aren't seeing it react with normal matter. So one potential explanation is a particle that is so tiny/fast, with no electrical charge, that it is can fly through planets without hitting anything: a "ghost" particle like the neutrino. The term used is WIMP (Weakly Interacting Massive Particle). Are we know is that it has mass, because it influences gravity. That's one possible explanation for dark matter.

Ideas based on extra dimensions are becoming less popular. We have evidence that gravity experiences the same dimensions that light does. Gravity waves from things like neutron stars merging seem to arrive at earth at the same time as light. So it is doubtful that extra dimensions can explain dark matter.