Our universe is a mysterious place. Only 4 percent or so is made up of the ordinary matter we see around us, including all those galaxies filled with stars. The rest, physicists believe, is made of up weakly interacting dark matter, and a mysterious substance called dark energy that is causing the cosmos to expand at an accelerating rate.
At least, that's the working hypothesis, and it seems to fit the data, although there are scientists who question its existence and tout alternatives to explain that accelerating expansion.
