Shoujo no Kami no Ryuushi

Those of the Dark Matter lived by swallowing others who did not accept them. In the midst of all this, Nemophila---the princess of the Dark Matter---fell in love with Sho, the sole survivor of a planet that she had destroyed for fun, and whom she'd spared as a joke. This is the story of how dark matter, which is "invisible" to everyone, came to acquire "mass". [Translated from official website]