The nature of dark matter
What is the invisible 'dark matter' that makes up most of the gravitating mass in galaxies and the universe, and can it be directly detected in a laboratory?
History and references
Still open in the literature as of 2026.
Ginzburg's problem #27 already treated dark matter's existence as established from galaxy rotation curves and cluster dynamics, with its identity unknown. Since then, evidence has piled up further: the Bullet Cluster (2006) showed gravitational lensing offset from visible gas, and Planck-era CMB data (2013-2018) pin its cosmic density precisely (about 27% of the universe). Yet its fundamental nature remains completely unknown: direct-detection experiments (XENONnT, LUX-ZEPLIN) have found no WIMP signal, with LZ's 2024 results excluding cross-sections down to 2.2x10^-48 cm^2 at 40 GeV, increasingly disfavoring the simplest WIMP paradigm; axion searches, sterile neutrinos, and other candidates remain under active investigation with no confirmed detection as of 2026.
Roughly five-sixths of all the matter in the universe is completely invisible and has never been directly touched by any experiment — we know it only by its gravity.
References
- What problems of physics and astrophysics seem now to be especially important and interesting (thirty years later, already on the verge of the XXI century)?, V L Ginzburg, Physics-Uspekhi 42 (4) 353-373, 1999
- A Direct Empirical Proof of the Existence of Dark Matter, D. Clowe et al., The Astrophysical Journal Letters 648, L109, 2006
- LZ Experiment Sets New Record in Search for Dark Matter, Berkeley Lab News Center (LUX-ZEPLIN Collaboration results), 2024
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