Could black holes exist to balance the expansion of the Universe?
If that were their 'function' they're failing at it. Because not only is the universe NOT slowing down (due to the combined gravitational effects of objects within it) its expansion rate is actually increasing! At during the course of the past several billion years, so they think, at least at this time. They base this on their direct observations of deep space objects and how fast they seem to be moving away from us, using various speed-measuring techniques.