Dark energy might have a partner that helps slow the growth of huge cosmic structures like superclusters. New analysis suggests unknown physics aids dark energy’s "antigravity" effect, counteracting gravity that builds these structures.

The universe’s large-scale structure—networks of galaxies, clusters, and superclusters forming the cosmic web—has been shaped by gravity over billions of years, but its growth is now slower than expected.

Using data from the Baryon Oscillation Spectroscopic Survey (BOSS), researchers detected signs that structure formation today is suppressed more than models predict, and this suppression appears independent of dark energy alone.

Dark energy, discovered in 1998, is thought to drive the accelerating expansion of the universe and makes up about 70% of its total matter-energy. The leading explanation is the cosmological constant (Λ) in the standard ΛCDM model, representing vacuum energy—energy of empty space linked to quantum fluctuations producing virtual particle pairs.

While vacuum energy sounds strange, it’s experimentally verified (e.g., Casimir effect). In ΛCDM, dark energy should be constant, but recent data from the Dark Energy Spectroscopic Instrument (DESI) suggests dark energy may change over time, contradicting ΛCDM predictions.

Chen and colleagues compared DESI and BOSS data and found the mystery deepened: the growth of cosmic structures is even less than expected, implying unknown physics beyond dark energy.

This suppression started roughly when dark energy began to dominate the universe, about 5 billion years ago, after matter had controlled cosmic expansion for billions of years post-Big Bang.

Though dark energy influences the expansion and slows structure formation, it alone cannot fully explain the reduced growth observed now. Statistical analysis shows the chance of these results being random is 1 in 300,000, suggesting either new physics or an unknown error in the data.

With upcoming data releases, the team plans to reexamine their findings to clarify these tensions challenging the standard ΛCDM cosmology.

Their research appears in Physical Review Letters.