Dark Matter and Dark Energy
Both are something we can't measure or observe (they're dark to us), but our equations don't balance if we don't pretend they're out there. (Dark Fluid is a unification of them). So we assume that 22% and 74% of the Universe (respectively): 96% of known space is something we just can't see -- or the universe doesn't make sense. But who said the Universe has to make sense? What if there are places where dimensions beyond our understanding are interacting with ours? Science is sometimes 3 blind men describing an elephant. They're each right, but that doesn't mean they get the gestalt (whole). And fudge factors like this, are more an excuse than an explanation.
This is also a religion. If you can't see, smell, touch, or taste it -- or observe or interact with it. You can't create experiments to test it. You just have to believe it is there... like God, or String Theory. If you only got 4% right on a test, and didn't know the other 96% of answers, that's not a passing grade, right? That's astrophysics in a nutshell: grading themselves on a very, very low curve.