The expansion of the universe is primarily driven by a property called dark energy, which is a hypothetical form of energy that is thought to permeate all of space and exert a repulsive gravitational effect. This repulsive force counteracts the gravitational pull of matter, including the gravitational attraction caused by neutron stars and black holes.
Neutron stars and black holes do possess strong gravitational forces due to their immense mass. However, their influence on the large-scale expansion of the universe is limited. The expansion of the universe occurs on cosmological scales, involving the collective motion of billions of galaxies over vast distances. The gravitational pull of individual objects, even neutron stars and black holes, is relatively localized and does not have a significant effect on the overall expansion.
On the other hand, dark energy is believed to be a property of space itself, causing it to stretch and accelerate the expansion of the universe. The precise nature of dark energy is not yet well understood, and it remains an active area of research in cosmology.
It's worth noting that the repulsive effect of dark energy becomes more pronounced as the universe expands, while the gravitational influence of matter weakens due to the increasing distances between galaxies. As a result, the expansion of the universe continues to dominate over local gravitational interactions, such as those exerted by neutron stars and black holes.