This portion of the universe generated from dark matter halos in the Uchuu simulation shows the distribution of galaxies and quasars up to billions of light-years away. The image shows different types of cosmic objects observed with DESI—bright galaxies, massive red galaxies, star-forming galaxies, and quasars—represented in different colours. A box enlarges the nearest region, where the cosmic web can be seen: the large-scale network of structures formed by galaxies. Although spectacular, this image corresponds to less than 0.1% of the total volume of the cosmological map, corresponding to three years of observations, recently published by DESI. Credits: E. Fernández-García et al. (2025). ‘DESI DR2 reference mocks: clustering results from Uchuu-BGS and LRG’
A team of cosmologists from the Institute of Astrophysics of Andalusia (IAA-CSIC) and the Institute of Astrophysics of the Canary Islands (IAC) has obtained the most accurate census to date of the dark matter halos of the Universe. The work is based on the development of a new model, called GPS+, capable of predicting how many dark matter halos exist at each stage of cosmic history.
In the universe, there are enormous invisible structures surrounding galaxies and galaxy clusters. These are dark matter halos, concentrations of matter that do not emit light and cannot be directly observed, but whose gravity holds galaxies together and guides their formation. These halos act as the "scaffolding" of the universe: galaxies form and evolve within them.
A new study led by the Institute of Astrophysics of Andalusia (IAA-CSIC) and the Institute of Astrophysics of the Canary Islands (IAC) has achieved the most precise census to date of these structures throughout the 13.8 billion-year history of the universe. This record, which cosmologists call the “halo mass function,” is not an individual list of objects, but a mathematical description that indicates how many dark matter halos exist in each mass range at a given epoch in the universe.
“This is important because not all halos are the same: some contain very small galaxies; others contain galaxies like the Milky Way; and the most massive ones can contain enormous clusters with hundreds or thousands of galaxies,” explains Elena Fernández García, a researcher at the IAA-CSIC and first author of the article, published in the journal Astronomy & Astrophysics Letters.
This new result is based on the development of a theoretical model called GPS+, which allows for highly accurate prediction of the abundance of dark matter halos at different stages in the history of the universe.
Towards a more precise description
This work represents a significant advance because it corrects limitations of previous approximations, which could deviate by up to 80% when describing the early universe. The new model reduces these discrepancies, especially at the mass extremes—where uncertainties were greatest—to around 10–20%, maintaining high accuracy throughout almost all of cosmic history.
“The key lies in a simple idea,” says Juan Bencort Rijo, a researcher at the IAC. “The matter in the universe doesn’t clump together to form perfect spheres, but rather irregular and complex structures. By incorporating this reality and other details of the gravitational collapse process, the GPS+ model more accurately describes how dark matter halos form and, consequently, how galaxies are born and evolve.”
To test the model's robustness, the team compared it with Uchuu—"universe" in Japanese—a set of the most comprehensive and accurate cosmological simulations to date. These simulations, in whose development the IAA-CSIC participated, were carried out by Tomoaki Ishiyama, a researcher at Chiba University and co-author of the study, and run on Fugaku, one of the world's most powerful supercomputers, in Japan.
“All the dark matter halo catalogs generated from the Uchuu simulations are available in our Skies & Universes database, developed at the IAA-CSIC,” says José Ruedas, head of this Big Data infrastructure and co-author of the work.
These simulations have not only served to test the model, but also to improve the tools used to interpret current astronomical observations. The new predictions will allow for more precise analysis of data obtained by telescopes such as the James Webb Space Telescope, which observes very distant galaxies formed during the early stages of the universe, as well as the results of large-scale sky surveys, such as DESI ( Dark Energy Spectroscopic Instrument ), whose objective is to reconstruct the large-scale distribution of matter in the universe and understand the nature of dark energy—an international project in which the IAA-CSIC has played a key role in its technological development and is currently in its scientific exploitation.
“Having a more accurate census of dark matter halos is key to connecting these observations with theoretical models and verifying whether our description of the universe —including the nature of dark matter and dark energy— fits the data,” says Elena Fernández (IAA-CSIC).
The GPS+ model is now available to the international scientific community, facilitating its incorporation into future analyses and simulations. This work reinforces the contribution of the Institute of Astrophysics of Andalusia (IAA-CSIC) and the Institute of Astrophysics of the Canary Islands (IAC) to cosmological research, in collaboration with Chiba University (Japan) and the University of Virginia (USA), and consolidates their participation in the development of essential theoretical tools for interpreting large-scale surveys of the universe.
Article: Elena Fernández-García et al. "A redshift-independent theoretical halo mass function validated with the Uchuu simulations", A&A, 707, L4 (2026). DOI: https://doi.org/10.1051/0004-6361/202558431
Contact at the IAC:
Juan Bencort Rijo, jbetanco [at] iac.es (jbetanco[at]iac[dot]es)
