Our universe is permeated with a vast, unseen force that seems to oppose gravity. Physicists call this force dark energy, and it is thought to be constantly pushing our universe outward.

But in June, a group of physicists published a paper in the preprint journal arXiv implying that dark energy changes over time. This means that the universe will not expand forever but might eventually collapse into the size it was before the Big Bang.

Almost immediately, however, physicists found problems with the theory: Several independent groups subsequently published papers that suggested revisions to the conjecture. Now, a paper published on Oct. 2 in the journal Physical Review D suggests that, as it stands, the original conjecture can’t be true because it can’t explain the existence of the Higgs boson — which we know exists, thanks to the Large Hadron Collider, the massive particle collider on the border between France and Switzerland. [Beyond Higgs: 5 Elusive Particles That May Lurk in the Universe].

Dark matter supposedly makes up 85% of the matter in the universe, but so far, efforts to catch hypothesized dark matter particles have all ended in failure. Weakly interacting massive particles (WIMPs) are no-shows at grand experiments housed in Italy, Canada, and the United States. Even tinier axions have not been detected either. Neutralinos, born out of supersymmetry, may look nice on paper but so far have no bearing on reality.

The standard model of modern cosmology is unthinkable without dark matter, although direct detections are still missing. A broad perspective of how dark matter was postulated and became accepted is presented, from prehistory, over observations of galaxy clusters, galaxy rotation curves, the search for baryonic dark matter, possible alternative explanations via modified gravity, up to the hunt for dark matter particles. The interplay is described between observational discoveries and theoretical arguments which led finally to the adoption of this paradigm.

Since the 1970s, astronomers and physicists have been gathering evidence for the presence in the universe of dark matter: a mysterious substance that manifests itself through its gravitational pull. However, despite much effort, none of the new particles proposed to explain dark matter have been discovered. In a review that was published in Nature this week, physicists Gianfranco Bertone (UvA) and Tim Tait (UvA and UC Irvine) argue that the time has come to broaden and diversify the experimental effort, and to incorporate astronomical surveys and gravitational wave observations in the quest for the nature of dark matter.

Over the past three decades, the search for dark matter has focused mostly on a class of particle candidates known as weakly interacting massive particles (or WIMPs). WIMPs appeared for a long time as a perfect dark matter candidate as they would be produced in the right amount in the early universe to explain dark matter, while at the same time they might alleviate some of the most fundamental problems in the physics of elementary particles, such as the large discrepancy between the energy scale of weak interactions and that of gravitational interactions.