What is it and why can't we detect it?
Dark matter is the most mysterious, non-interacting substance in the Universe. Its gravitational effects are necessary to explain the rotation of galaxies, the motions of clusters, and the largest scale-structure in the entire Universe. Many galaxies and other cosmic structures are spinning too fast to be able to hold together everything they contain; all the stuff they contain should be flying apart. The only way to explain their high spin rate is if these cosmic structures have a lot more mass than we detect.
But many experiments to directly detect dark matter have failed. In other words, we have a name -- dark matter -- for something that we really don't know even exists.
There is a solution, though, that does not require dark matter. The stars we see are the way they looked in the past. So, what if mass was heavier, and therefore had stronger gravity, in the past? That would solve the problem. The V-Bang Theory explains, among other things, why matter in the past was a lot heavier than it is today, and there is no need to speculate about some mysterious, undetectable dark matter to explain the spin rate of cosmic structures.