1550 supernova flashes and millions of galaxies — that's the statistics of the DESI survey. This dataset allowed, for the first time, a suspicion: the repulsive dark energy has a hidden counterweight. The cosmic accelerator is gaining momentum, but somewhere deep inside hides an invisible brake — a negative cosmological constant (vacuum energy that attracts rather than repels).

The story of dark energy began in 1929, when Edwin Hubble measured the expansion of the universe. In 1998, Adam Riess and his colleagues noticed that this expansion is accelerating. The prime suspect became the dark energy Λ — an unchanging vacuum energy that pushes space apart. But the latest data from DESI on baryon acoustic oscillations (density waves frozen after the Big Bang) showed: Λ might not be constant. Theorists have long suggested that in string theory, anti-de Sitter vacua — states with negative energy — are natural. Thus arose the hypothesis: a negative Λ could coexist with the familiar dark energy.

The researchers tested this hypothesis with two methods. The DESI data were divided by redshift (the stretching of light due to the expansion of space) into 10 bins. In parallel, they applied Gaussian processes — a statistical tool that draws smooth curves without a predefined formula. Both approaches, after analyzing 1550 supernovae from the Pantheon catalog, gave a weak preference for a model with a negative component. The significance level is around 1σ. The counterweight density turned out to be −0,093, but with large uncertainties (+0,525/−0,294). For now, it's just a hint.

Data from the cosmic microwave background have not yet been included, but it is precisely these that will help separate the counterweight from other effects. The main surprise is the behavior of the equation of state w(z) (the relation between pressure and density). Without the counterweight, part of the data would push w into the phantom regime (w < −1). There, energy would grow to infinity. The cosmos would be torn apart in a Big Rip. The negative component brings w back to a 'calm' quintessence. It keeps the parameter within 1σ of w = −1. The counterweight seems to hold back dark energy, preventing it from crossing the forbidden line.

The result does not resolve the Hubble tension — a discrepancy in measurements of the expansion rate. But it opens a path to alleviating this contradiction in tandem with early dark energy. Future data from DESI and the Euclid mission will clarify whether the counterweight is real. For now, astrophysicists are peering into redshifts of 1–2, where the influence of negative energy is most noticeable. The cosmic accelerator continues its runaway. And perhaps, behind its rapid dash, an invisible brake is watching.