Dark matter is the unseen material whose gravity holds galaxies and galaxy clusters together, and somewhere between 90 and 99 percent of the universe's material is in this undiscovered form, according to an encyclopedia entry on the two leading candidates. For decades, scientists pursued two very different explanations: WIMPs, exotic subatomic particles, and MACHOs, dim ordinary objects like dead stars. The verdict so far is lopsided. MACHOs have been largely ruled out as the dominant dark matter because there simply are not enough of them, while the search for WIMPs continues in underground detectors and particle accelerators.
The Dark Matter Enigma: WIMPs and MACHOs
Why do we need dark matter at all? Galaxies rotate and cluster as though they contain far more mass than we can see. The missing material acts as gravitational glue, and it also shapes the history and fate of the universe. But it neither emits nor absorbs light, so it cannot be observed directly.
Two broad answers emerged. One says the missing mass is made of ordinary matter that is simply too dark to see: compact objects such as brown dwarfs, neutron stars, white dwarfs, or even primordial black holes scattered through galactic halos. These are the MACHOs, or Massive Astrophysical Compact Halo Objects. The other says dark matter is something entirely new: heavy, weakly interacting particles outside the known Standard Model of physics. These are the WIMPs. As an astronomy encyclopedia explains, the two represent "two very different but reasonable possibilities of what the dominant component of the universe may be."
Weakly Interacting Massive Particles (WIMPs): A Particle Physics Solution
WIMPs are a hypothesized class of particles, not yet observed. NASA's Scientific Visualization Studio describes them as particles that neither absorb nor emit light and do not interact strongly with other matter. When two WIMPs encounter each other, they annihilate and produce gamma rays, a signature that experiments can look for.
Their theoretical appeal comes from particle physics. Extensions of the Standard Model, notably supersymmetry, predict hundreds of candidate particles that could fit the dark matter role, including neutralinos, photinos, higgsinos, sneutrinos, and new heavy neutrinos. Writing in The Oxford Scientist, Tessa Hayhurst notes that WIMPs are thought to be stable and slow moving, meaning they may have existed since the universe formed. That stability makes them plausible building blocks of cosmic structure.
The catch is their elusiveness. Because WIMPs interact so rarely, any experiment hoping to catch one must be extraordinarily sensitive. They remain, as Hayhurst puts it, "currently purely hypothetical."
Massive Astrophysical Compact Halo Objects (MACHOs): An Astronomical Approach
MACHOs take the opposite approach: no new physics required. They are baryonic, meaning they are made of the same ordinary matter as stars and planets, but they are invisible or extremely dim. Candidates include brown dwarfs (failed stars), neutron stars, white dwarfs, and primordial black holes.
The idea is simple in principle. If a galactic halo were filled with Jupiter-mass objects, the encyclopedia entry notes, they would have escaped detection by the emission or absorption of light, because such objects shine far too faintly to be seen. Yet their mass would still exert gravity, exactly what dark matter observations demand. MACHOs are therefore considered a form of cold dark matter: massive, slow-moving material that reveals itself only through its gravitational pull.
Detecting the Invisible: Experimental Searches for WIMPs and MACHOs
Because the two candidates are so different, the searches for them look nothing alike. Hunting WIMPs is a particle physics problem. One strategy is to create them in particle accelerators, smashing particles together with enough energy to produce new heavy particles. Another is to sense them directly in underground detectors as they pass through the Earth and, very occasionally, collide with an atomic nucleus. A third, indirect approach watches the sky for the gamma rays produced when WIMPs annihilate one another.
Hunting MACHOs, by contrast, is an astronomical observation problem. Since these objects are too dim to see, astronomers rely on gravitational microlensing: when a MACHO passes in front of a background star, its gravity briefly bends and brightens that star's light. Monitoring millions of stars for these fleeting brightening events reveals how many compact objects are drifting through the halo.
The underground WIMP effort is active and increasingly sensitive. Researchers with the LUX-ZEPLIN experiment at the Sanford Underground Research Facility in South Dakota announced on September 1, 2026 a single possible collision between a xenon nucleus and a dark matter particle with a mass of at least 200 gigaelectronvolts, and probably around 1,000 GeV, a candidate event that would still require confirmation.
WIMPs vs. MACHOs: A Comparative Overview
The table below contrasts the two candidates across the dimensions that define them. Use it as a quick reference: each row captures a distinction developed in the sections above, from what the candidates are made of to how scientists search for them.
| Dimension | WIMPs | MACHOs |
|---|---|---|
| Category of matter | Non-baryonic; particles outside the Standard Model | Baryonic; made of ordinary matter |
| Hypothesized composition | Supersymmetric particles such as neutralinos, photinos, higgsinos and sneutrinos, plus new heavy neutrinos | Brown dwarfs, neutron stars, white dwarfs, primordial black holes, and Jupiter-mass objects |
| Theoretical origin | Extensions of the Standard Model, such as supersymmetry | Known types of astronomical objects too dim to observe directly |
| Primary detection method | Direct detection of nucleus collisions in underground detectors; creation in accelerators; indirect search for annihilation gamma rays | Gravitational microlensing, watching background stars brighten as a MACHO passes in front |
| Current status | Still the leading pursued candidate, but purely hypothetical and undetected | Largely ruled out as the dominant dark matter; too few objects observed |
Why MACHOs Fell Out of Favor
MACHOs were attractive because they required no new physics. But the observational verdict has been unfavorable. As Hayhurst reports, though MACHOs are good candidates in principle, "there are not nearly enough of them to account for its predicted total mass." Microlensing searches did find compact halo objects, but the tally falls far short of the mass dark matter requires.
That shortfall matters because dark matter dominates the universe's material content. If MACHOs made up the difference, microlensing surveys would have seen abundant brightening events. They did not. The result shifted scientific attention to the second category: WIMPs, which remain hypothetical but are still being pursued with accelerators, underground detectors, and gamma-ray searches. Other candidates, such as axions, are also under investigation, though WIMPs and MACHOs remain the classic contrast.
The Shifting Landscape of Dark Matter Research
Scientists have largely ruled out MACHOs as the primary dark matter candidate due to insufficient observational evidence from microlensing surveys, while the search for WIMPs continues with ongoing direct and indirect detection experiments. For readers following the field, the measurable indicator to watch is continued funding and publication of results from WIMP detection experiments such as LUX-ZEPLIN and XENONnT, or announcements of new constraints on WIMP properties. The September 2026 LUX-ZEPLIN candidate event shows the search is live: the next published result will either strengthen that signal or tighten the limits on what a WIMP can be.
