So, why would one object affect another over a distance, in a vacuum? We see this in the gravitational affect of our planet, the sun and the moon. We see it in the forces of a magnet. We know it happens at the atomic level as atoms and molecules don't actually collide, but rather influence each other from a distance.
Perhaps we could imagine some dark matter secret hooks in particles as tiny as atoms. But what about as far as the sun is from Pluto?
Cosmologists looked at galaxies and expected to see the outer rim to trail behind star that were closer to the core, in the same way that our outer planets trail behind the inner planets. (Once around the sun is a year for the earth, but several years for planets further out.) But they found that the outer rim of many galaxies doesn't trail, actually taking the same time to revolve around the galactic center as do stars closer in.
They decided that dark matter must be the reason. Oh they've proven it mathematically. But what is it? How can the apparent vacuum of space contain something which causes atoms in proximity to react to each other, and planets at great distances to be caught in this unseen link?
Gravity is a force. But what is a force that it can act in a vacuum at a distance?
