The universe is accelerating away from us, but we don't know why

Space · Published 22 October 2026 Swipe more like this →

In short: For billions of years, gravity acted as the universe's brake, slowing down its expansion. Then, something changed. About nine billion years after the Big Bang, an unknown force began pushing everything apart faster and faster. This mysterious driver makes up nearly 70% of everything in existence.

Dark energy was not discovered until the late 1990s, though its roots lie in early 20th-century astronomy. In 1912, Henrietta Swan Leavitt established a relationship between the period and luminosity of Cepheid variables, allowing for precise distance measurements. Around that same time, Vesto Slipher used a spectrograph to observe redshift—the stretching of light waves as galaxies move away from us. These foundational tools eventually allowed scientists to realize that distant galaxies are not just receding, but are accelerating in their retreat.

Albert Einstein's general relativity theory introduced the cosmological constant, which is thought to be the main factor behind this speeding up. Dark energy is the term frequently used by contemporary cosmologists to describe this phenomenon. It is characterized by an unusual property of negative pressure, which allows it to overcome the attractive force of gravity on a cosmic scale. While it remains a mystery, it is a core component of the Lambda Cold Dark Matter (ΛCDM) model, which explains the large-scale structure of the universe.

Despite its dominance, dark energy cannot be explained by the standard model of particle physics. Its existence is supported by multiple lines of evidence, including the study of Type Ia supernovae—exploding white dwarf stars with known brightness—and fluctuations in the cosmic microwave background. However, the ΛCDM model faces increasing scrutiny. Some critics argue that its successes may stem from confirmation bias, noting that all current analyses rely on a specific solution to Einstein's equations known as the FLRW framework.

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