Dark matter is known to exist in other dimensions and can only interact with the matter on our dimension through gravity. The dark matter located on different dimensions can aggregate in the same way as the matter in our visible universe does, forming exotic galaxies. Multidimensional theories explain that the unique force of gravity has an effect across all the various extra dimensions, which accounts for the relative weakness of the force of gravity, compared to the other known forces of nature (electromagnetism, strong interaction, & weak interaction) – those that cannot cross into all the higher dimensions.
What is called antimatter is also receiving fresh attention, and a Riken article about it has been neatly condensed this week at the following website: http://thewatchers.adorraeli.com/2015/08/21/new-research-reveals-matter-and-antimatter-mirror-images-of-each-other/ Following is taken from that story, showing that gravity also acts upon this form of unseen matter:-
This month, researchers at CERN have conducted the most precise measurement so far of the charge-to-mass ratio of protons and their antimatter counterparts, antiprotons. The scientists got their results during a stringent test of a fundamental property of the standard model of particle physics, known as CPT (charge, parity, time) symmetry. The experiment was set to test if the system would remain unchanged when the fundamental properties responsible for distinguishing between matter and antimatter were reversed.
The team used the results to show that within approximately one part per million, antimatter and matter behave in the same way in respect to gravity. "This is an important issue, because it helps us to understand why we live in a universe that has practically no antimatter, despite the fact that the Big Bang must have led to the creation of both," explained Stefan Ulmer, the leader of the research. "There are many reasons to believe in physics beyond the standard model, including the mystery of dark matter and, of course, the imbalance between matter and antimatter," team member Christian Smorra added.