Okay. Gravity is as yet unseen. So is dark matter. How does gravity work, and attract normal matter? Check out this article:
http://news.vanderbilt.edu/2013/06/dark-matter/
“… the physicists propose that dark matter, an invisible form of matter that makes up 85 percent of the all the matter in the universe, may be made out of a type of basic particle called the Majorana fermion. The particle’s existence was predicted in the 1930’s but has stubbornly resisted detection.
A number of physicists have suggested that dark matter is made from Majorana particles, but Scherrer and Ho have performed detailed calculations that demonstrate that these particles are uniquely suited to possess a rare, donut-shaped type of electromagnetic field called an anapole. This field gives them properties that differ from those of particles that possess the more common fields possessing two poles (north and south, positive and negative) and explains why they are so difficult to detect.”
“… Scientists hypothesize that dark matter cannot be seen in telescopes because it does not interact very strongly with light and other electromagnetic radiation. In fact, astronomical observations have basically ruled out the possibility that dark matter particles carry electrical charges.”
There’s a lot of work being done over the last fifty years trying to figure out this stuff. A lot has been written. Einstein couldn’t figure it out. But a breakthrough could be imminent (I hope).
It’s been said for a long time that dark matter could be something similar to the neutrino, but with more mass. Neutrinos can’t clump near galaxy clusters because they just fly away at the speed of light. It’s also been suggested that dark matter may not even exist; we just don’t fully understand how gravity works over long distances. I was reading an article about dark matter once (my doctor just happened to have a Scientific American mag in his waiting room), that claimed if dark matter exists, we are in a desert zone of it. Apparently they can’t find any trace of its gravitational influence in the surrounding stars out to a few thousand light years.