| Humans have wondered where the world came from for millennia, but it's only in the last century that we've started to figure it out. Edwin Hubble's observations in the 1920s revealed that our Milky Way is just one of many galaxies, and that these galaxies are all being dragged away from one another by the expansion of space. That led cosmologists to infer that the universe is not eternal — that an explosive "Big Bang" in the distant past had kicked off the expansion.
In the last 30 years, cosmologists have pinned down this cosmic origin story with precise numbers. In 1998, researchers monitoring the motion of dozens of galaxies discovered that the expansion of space is ever so slightly picking up its pace, an acceleration powered by a mysterious "dark energy." Then, in the 2000s, a NASA satellite mapped out the leftover heat glow of the early universe in sufficient detail for researchers to deduce when the universe began (13.77 billion years ago) and what its main ingredients are today (71.4% dark energy, 24% dark matter — an invisible form of matter that holds galaxies together — and 4.6% visible matter). Adam Riess, one of the Nobel Prize–winning discoverers of dark energy, called this moment "the end of the beginning in our quest to understand the Universe."
In recent years, the quest has matured and intensified. Researchers have measured the same heat glow much more precisely from both the ground and space. And where cosmologists once made inferences based on the motions of dozens of galaxies, they now have access to millions.
But as the mountains of data have accumulated, cracks in cosmologists' accounting have begun to emerge — conflicts between theory and data that they refer to affectionately and hopefully as "tensions."
Tensions both invigorate and frustrate the field. They may foreshadow big discoveries. Or they may indicate subtle missteps in analysis that crop up as cosmologists attempt to integrate millions of observations of wildly different cosmological objects.
Answers are on the way. A new wave of flagship observatories is coming online, and their data seems likely to relieve the field's growing tensions, one way or the other.
What's New and Noteworthy
The oldest and sharpest tension, called the Hubble tension, has grown so tense in recent years that some cosmologists have taken to calling it a "crisis." When they look at nearby galaxies, they find that the universe is expanding at a certain speed. But when they forecast the expansion rate based on the heat glow from the early universe, they get a slower speed. Something is missing. It could be an ingredient in the theoretical forecast, such as an "early" form of dark energy that animated the young universe. Or it could be a more mundane detail throwing off modern observations, such as dust clouds deceptively dimming parts of nearby galaxies. The James Webb Space Telescope recently turned its eye on the problem, but the debate continues to rage.
In 2020, a second anomaly got cosmologists fired up. Their best forecast suggests that, over the last 13.77 billion years, gravity should have pulled dark matter together into clumps of a certain average size. But scientists with the Kilo-Degree Survey (KiDS) collected light streaming from 31 million galaxies and found that the dark matter between us and those galaxies seemed more spread out than expected. This "sigma-8 tension," however, all but evaporated in March 2025. A new analysis of the final KiDS data set, which contained an additional 10 million galaxies, indicated that dark matter has the expected degree of clumpiness after all. Maps of dark matter constructed in other ways continue to show some modest tension with the forecast, but hope that the sigma-8 tension is significant has dimmed.
The newest tension hit cosmology in a bombshell finding last spring. The standard picture of dark energy sees it as the background energy of space itself, in which case it should be unwavering in strength. Indeed, since 1998, survey after survey has suggested that dark energy has maintained a constant intensity over the eons. But when the Dark Energy Spectroscopy Instrument (DESI) mapped out the locations and speeds of millions of galaxies, it found subtle hints that dark energy may be diminishing — a confounding result that only theorists specializing in string theory find natural. An update this spring from DESI and the Dark Energy Survey (DES), a collaboration that surveyed the sky with a different telescope, found that the evidence for weakening dark energy has grown somewhat stronger.
As the KiDS and DES collaborations publish final analyses of their full data sets this year, the so-called Stage III era of cosmology is winding down. DESI represents the first of the next-generation Stage IV observatories. Another Stage IV instrument, the hotly anticipated Vera Rubin Observatory, will switch on in July and start surveying the sky with the largest digital camera ever constructed.
The tensions might all melt away, confirming the maturity of our current theory. If they don't, Stage IV may mark the beginning of a whole new chapter in our quest to understand the cosmos. | 
