This possibility has been seriously considered by modern science, prompted by a number of facts. Perhaps the most notable one is that if we calculate the size of a black hole with mass and energy equal to all the mass and energy we can see in the Universe, we get a surprising result: it’s almost the same size of the observable Universe!
If this seems odd, remember that a black hole’s radius is directly proportional to its mass, while its volume is proportional to its radius cubed. So, the more massive a black hole is, the less dense it is. So a black hole with the Universe’s radius would have roughly the average density we observe in the Universe.