It's always a dicey thing to try to simplify a complex scientific idea, but here's my roll of the dice on the subject of dark matter.
With the given that stuff in the universe is held in proximity to other stuff by the force of gravity - a force inherit in all stuff - scientists have been scratching their collective heads for a while because there appears to be more gravity than can be explained by the amount of stuff.
A quote from schools-wikipedia.org:
The first to provide evidence and infer the existence of a phenomenon that has come to be called "dark matter" was Swiss astrophysicist Fritz Zwicky, of the California Institute of Technology (Caltech) in 1933. He applied the virial theorem to the Coma cluster of galaxies and obtained evidence of unseen mass. Zwicky estimated the cluster's total mass based on the motions of galaxies near its edge. When he compared this mass estimate to one based on the number of galaxies and total brightness of the cluster, he found that there was about 400 times more mass than expected. The gravity of the visible galaxies in the cluster would be far too small for such fast orbits, so something extra was required. This is known as the "missing mass problem". Based on these conclusions, Zwicky inferred that there must be some non-visible form of matter which would provide enough of the mass and gravity to hold the cluster together.
Translation: We can't explain why the universe doesn't just blow apart. There isn't anything natural/physical there that we can detect, so for now, let's call it "dark matter" and say that we haven't found a way to determine exactly what it is...but we can see its effect in the gravitational forces that are evident.
Okay...I read all that...then I turn to Colossians 1:17...
"[Jesus] is before all things, and in him all things hold together."
Truth is...I think maybe the Apostle Paul has had the answer all along.