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Invisible matter, invisible brains? Andy Lawrence https://orcid.org/0000-0002-3134-6093Authors Info & Affiliations Science 27 Feb 2025 Vol 387, Issue 6737 1,095 Metrics Total Downloads 1,095 Last 6 Months 1,095 Last 12 Months 1,095 eLetters (0) Information & Authors Metrics & Citations Share Cosmology finds itself in a paradoxical situation. We know everything and nothing. After years of accumulating evidence and debate, there is a consensus model about the makeup of the cosmos, known as Lambda Cold Dark Matter (ΛCDM), for reasons explored below. The Universe seems to have no spatial curvature, is clumpy on small scales but smooth on large scales, and is expanding at a well-measured but accelerating rate. The contents of the Universe are divided between ordinary matter, dark matter, and dark energy. This phenomenological model is exquisitely constrained by observations, but it is not known what dark matter and dark energy actually are or why the Universe is this way. Only 5% of the mass energy in the Universe is baryonic matter, the ordinary stuff of which the stars are made. There can't be more baryons than this, or nucleosynthesis in the dense hot early universe would have made more light elements than we see. Another 27% is “dark matter,” consisting of some kind of mystery particle formed in the swirling maelstrom of the early Universe that then “froze out” as the Universe expanded and cooled. This particle neither emits, absorbs, nor reflects light, hence the “dark” in ΛCDM, but we see its gravitational effects. It is known that this particle must be moderately massive and thus slow moving, hence “cold” in ΛCDM. Light, fast-moving particles would smooth out small-scale structure in a manner that we don't see. There are many theoretical proposals for what the mystery particle is, but nobody knows. SIGN UP FOR FIRST RELEASE ALERTS Get the latest First Release papers from Science delivered right to you The other 68% of the Universe is “dark energy,” an energy inherent in the vacuum that drives the accelerating expansion. The nature of the dark energy is characterized by a value known as the equation of state parameter (w). The value of this parameter is tightly constrained by combining several distinct types of measurement, the degree of clumping of matter, the distortions caused by gravitational bending of light, direct measurement of the expansion rate at various epochs using supernovae as so-called standard candles, and precise measurement of the wiggles in the Cosmic Microwave Background, the oldest and most distant light in the universe that we can detect, which arose after the Big Bang. These distinct measurements are all consistent with the simplest possible value, w = –1, corresponding to a “cosmological constant” (Λ) in traditional algebraic formulations of cosmology. However, the model is phenomenological; it explains what we see, but not why it is this way. Although there are well-known theories, we have no idea what the dark energy is. How do we escape this unsettling situation, where effects of the dark sector are well measured, but astronomical observations do not explain its fundamental nature? One hope is to make a direct detection of the dark particles on Earth. The dark matter particles should be streaming past us at several hundred kilometers per second. They interact rarely with normal matter, but every so often, a collision should produce a recoil that we can detect. This is a very difficult measurement to make because of the constant noise of many other everyday events, so dark matter detectors are usually hidden underground. After several decades of trying, nothing has been seen, but the experiments get ever better. Hopefully, there will be progress soon. Another hope of some astronomers is that the dark sector doesn't exist at all; they hypothesize that what is really going on is that gravity is not completely described