They've looked and looked and looked for dark matter and they can't find it. The reason they can't find it is the unexplainable movements of objects in the universe are caused by a strong electrical force that they're not accounting for. So since they don't account for the strong electrical force they have to make up a force called dark matter or rather a material cause called dark matter which they assume exerts a gravitational force but they've looked and looked and looked and they can't find it because it's not there.

does exist, shows in the name, dark, it means something they know nothing about like the dark ages

In this mirror universe, Thalidomide babies never happened.