The existence of dark energy is inferred from observations and measurements of the expansion of the universe. While we may not know what the universe is expanding into, we can still study the rate at which it is expanding.
In the late 1990s, astronomers made a remarkable discovery based on observations of distant supernovae (exploding stars). They found that the expansion of the universe is accelerating rather than slowing down, as previously expected. This unexpected acceleration implied the presence of a mysterious force, now called dark energy, which is driving the expansion.
Dark energy is a hypothetical form of energy that permeates the entire universe and has negative pressure. It acts as a repulsive force, counteracting the gravitational attraction between matter and causing the accelerated expansion.
Other observations, such as the cosmic microwave background radiation and large-scale structure of the universe, also support the existence of dark energy. These observations provide additional pieces of evidence that are consistent with the idea of dark energy.
While we don't yet fully understand the nature of dark energy or what it fundamentally is, its presence is inferred based on its effects on the expansion of the universe. Future research and observations are aimed at gaining a deeper understanding of dark energy and its role in shaping the universe's evolution.