Final answer:
The expansion of the universe is accelerating due to dark energy, which overpowers the gravitational pull between galaxies. Dark energy's presence is inferred from observations and is represented by the cosmological constant in the equations of general relativity, driving the observed increase in the universe's expansion rate.
Step-by-step explanation:
The expansion of the universe appears to be accelerating, and this is largely attributed to the cosmological constant or, more generally, dark energy. While gravitational attraction between galaxies works to slow this expansion, observations show that the universe is expanding at an increasing rate, indicating that dark energy is overcoming gravitational forces.
The cosmological constant represents a repulsive force that counteracts gravity in Einstein's equations of general relativity. As the universe grows and the matter within it thins out, dark energy becomes more significant, leading to the observed acceleration of the expansion. This ongoing "Cosmic Tug of War" between the forces of gravity and dark energy will ultimately determine the fate of the universe.