Dark Matter 「暗黒物質（ダークマター）」 is a type of matter hypothesized in astronomy and cosmology to account for a large part of the mass that appears to be missing from the universe.
To easily understand what Dark Matter is, it's something like..
Mass of the Universe = earth + all other stars + all other planets + gases + various other things + ???
this "???" = the dark matter)
This term is related to Aoi's Coppelion power as mentioned by Mana and Dr.Coppelius. In the latest chapters, Ibara wonders if this Dark Matter has something related to Setsuna's Spacial Displacement abilities.