That came from the 'expanding universe' theory, which has a giant flaw.
In order for the 'universe' to expand and yet keep the same 'density', it must somehow acquire MORE MASS.
Based on estimates of the size of the universe (which really only reflects our limited ability to perceive), if it were expanding, it would require more mass than we estimate is IN the ENTIRE KNOWN universe.
Where has it been asserted that the universe is keeping the same "density"? As space expands, the average "density" of the universe goes down, obviously.