We think we have accounted for all the ordinary energy in stars, galaxies, intergalactic dust, etc., but things at the very largest scales we can observe, galaxies seem to be drifting apart from each other. Our model of physics tells us that the energy (which is also mass; e=mc^2 mass and energy are equivalent 'things,' just in different forms) we're familiar with is gravitationally attractive, so we surmise there must be some sort of exotic repulsive energy counteracting the attractive stuff we've accounted for. And if we use our model to estimate how much of this so-called 'dark energy' there is pushing the galaxies away from each other (based on things like galactic mass, apparent rates of recession, and so on), we get that there is something like 15 times more of this 'stuff' pushing things apart than there are actual things. We dont know what this exotic energy might be, we cannot confirm our hypothesis because no way of detecting this stuff in a lab has ever been devised. It's entirely possible our model is simply flawed, or that our observations are incorrect or incomplete.