In modern physics, field equations within the standard model predict that there is a certain amount of matter (mass) out there in the universe. However, when looking for the predicted mass with telescopes and other advanced technology, it was realized that most of the mass that was predicted to be there was actually "missing", not just a little bit of it... 96% of the matter was missing(!)
At this point you might expect a reexamining and consequential adjustment in the predictive model, instead, a new type of matter was invented "out of thin air" in hopes it could make all the equations work out. They needed to come up with a highly technical term for this new type of invented matter and since they couldn't seem to find it anywhere they decided to call it.... "dark matter" and "dark energy".
Perhaps the mass is "missing" because of another fundamental error in the standard model, one that that got rid of a rather incredibly large (approaching infinite) number that represents how dense empty space actually is. Most physics students are not even aware of this mathematical trick called "renormalization" where the density of space was effectively ignored despite the scientifically verified through experiment (the Casmir effect) density of space-time itself in peer reviewed and published laboratory experiments.
Perhaps we are getting closer to the point where the standard model realizes there is no such thing as "dark matter" and that the missing mass is in fact the energy within the structure of the space-time manifold itself, something that Nassim Haramein has predicted in his unified field theory model for many years.