🌀 The Andromeda galaxy anomaly resolved

The expansion of the Universe, discovered nearly a century ago by Edwin Hubble, shows that galaxies are moving away from us faster the more distant they are. Yet, our nearest neighbor, the Andromeda galaxy, is approaching the Milky Way at 100 km/s (about 62 miles/s).

As early as 1959, this anomaly led astronomers to postulate the existence of dark matter to explain this motion. But a paradox remained: why does cosmic expansion around us appear so little disturbed, despite the presence of massive galaxies like Andromeda? A European team, including researchers from the IAP, has just solved this enigma by revealing the hidden structure of our cosmic environment: matter is organized into a thin flattened layer, leaving vast areas almost empty.


To solve this mystery, the researchers used advanced numerical simulations. Their algorithms reconstructed the initial conditions of the early Universe, making them coincide with the observed properties of the cosmic microwave background, and constraining that the Milky Way and the Andromeda galaxy today occupy the measured positions and velocities. The analysis of several hundred scenarios compatible with the data highlights a recurrent configuration: a mass distribution shaped like a "sheet," stretching well beyond our group of galaxies.

The results show that this structure simultaneously explains the weak perturbation relative to expansion and Andromeda's motion, thanks to a gravitational equilibrium between the central galaxies and adjacent empty regions. This discovery, as unexpected as it is elegant, reveals a structuring of our nearby Universe that is far more peculiar than imagined. It opens perspectives on how the cosmic environment shapes galaxies, including those in our immediate neighborhood.