| Through increasingly precise observations, astrophysicists have learned that all the stars and galaxies in the universe are a bit like city lights viewed from space; they imply the existence of vast, unseen continents below. Fritz Zwicky, Vera Rubin and other 20th-century astronomers discovered the first of two invisible continents while observing the motions of stars and galaxies. For outer stars to whip around the center of a galaxy as quickly as they do, Rubin and her colleagues realized, they must be held by the gravity of something invisible. The galaxy's bright spiral would have to be a small seed sitting in the center of an unseen cloud of "dark matter." Further evidence for this nonluminous form of matter appeared when physicists began to scrutinize ancient light from the early universe. They discerned ripples set off by a struggle between dark matter and visible radiation. Today, astronomers measure the distribution of dark matter from how it distorts light, much as you can tell where the surface of your drink is based on how it distorts the appearance of your straw. Two teams of cosmologists simultaneously stumbled upon the second dark continent in the 1990s. The movement of distant supernovas revealed that the expansion of the universe was picking up speed. Some sort of "dark energy" was working against gravity to drive galaxies apart faster and faster. Theorists immediately identified the prime suspect: a tiny amount of energy intrinsic to the vacuum of space. Albert Einstein had long ago considered this possibility, dubbing it a "cosmological constant." Such energy would have had a negligible effect early on when the universe was small, but its effect — repulsion — would grow with the size of the universe. In the very long run, the increasingly rapid expansion is expected to dilute visible and dark matter to imperceptible levels, leaving an explosively expanding universe made up almost exclusively of dark energy. These developments culminated in an overarching cosmological theory known as the Lambda-CDM model, a set of ingredients and equations describing the evolution of the cosmos. Lambda is a Greek symbol referring to dark energy, which currently accounts for 70% of the universe's energy. CDM stands for "cold dark matter," which makes up 25% of the cosmos's energy (via Einstein's equivalence of mass and energy). Visible matter, such as atoms — the last 5% of the cosmic pie chart — doesn't make it into the theory's name but is included in its equations.
What's New and Noteworthy In labs across the globe, physicists have tried to identify the elusive particle, or particles, that seem to be holding galaxies together. The hope is that dark matter isn't perfectly dark, and that it might occasionally light up an appropriately tuned detector. Increasingly heroic efforts to detect one promising candidate, a hypothetical particle dubbed the WIMP, have now gone on for decades. But the eureka moment hasn't come, so in recent years the search has expanded to cover a wider range of candidate particles. An alternative possibility is that dark matter is an illusion because gravity has some quirks we don't fully understand. In 2020, theoretical physicists managed to bend Einstein's theory of gravity to produce a few of the observed effects of dark matter. They pulled it off by adding a carefully crafted field, which they described as "dark dust," that acted a lot like matter. While there are many plausible ideas for what dark matter might be, dark energy has proved more troubling to the foundations of theoretical physics. Its tiny density is an especially huge mystery. A naïve estimate of the vacuum energy based on adding up the base-level energies of all the quantum fields that fill space gives a number that's gigantic compared to the tiny source of cosmic acceleration, a mismatch sometimes referred to as the worst prediction in physics. The best explanation for dark energy's dimness is also widely reviled. Perhaps, some physicists suspect, our universe is one bubble in a vast multiverse, where each bubble has a randomly determined vacuum energy. We live in a bubble with a minuscule vacuum energy because if its value were any larger, as Nobel laureate Steven Weinberg pointed out in 1987, the universe would have expanded too quickly for galaxies to form and (therefore) for life to arise. In this picture, the vacuum energy would be a bit like Earth's distance from the sun; it could have come out any number of ways — and indeed for other planets or other universes, it did — but only certain outcomes will be habitable. Weinberg's argument came to weigh heavily on many physicists. It places a major feature of our reality beyond the reach of calculation. Nima Arkani-Hamed, a physicist at the Institute for Advanced Study in Princeton, New Jersey, told me on a recent reporting trip that Weinberg's argument, when he heard about it, "hit me like a ton of bricks. I spent a month wandering around in a daze." Theorists have made some attempts to turn the multiverse hypothesis into a predictive theory, but multiverse abhorrence largely continues today. Meanwhile, observers have plowed ahead, tracking the effect of dark energy throughout cosmic history in ever-increasing detail. Their surveys of the cosmos have usually confirmed Lambda-CDM and the idea of dark energy as a cosmological constant. Just a few weeks ago, however, a team announced a tantalizing new clue to dark energy's nature. Analyzing thousands of supernovas together with subtle ripples in the distribution of 6 million galaxies, the Dark Energy Spectroscopic Instrument (DESI) collaboration found a hint that dark energy might be weakening over time. The anomaly could be a statistical fluke, or indicative of a misunderstanding about the astrophysics of supernovas. If so, further data from the DESI team should resolve the situation in the next few years. But if that hint does become a discovery, its implications will be profound. To some string theorists, it's natural to expect that our universe's vacuum energy might be dropping, since universes with constant positive vacuum energies appear difficult to construct in string theory. Others, however, stress that it would only create more headaches. Physicists would have to explain not only why the vacuum energy is improbably tiny, but also why it's changing improbably slowly. "If dark energy is really weakening," Daniel Harlow jokingly told me on a recent visit to the Massachusetts Institute of Technology, "I'll quit physics and go into tech." | 
