“You can journey to the ends of the earth in search of success,” 19th-century Baptist preacher Russell Conwell is said to have proclaimed, “but if you’re lucky, you will discover happiness in your own backyard.”
Modern cosmology has stepped far beyond our cosmic backyard. We peer into the light from the earliest moments of the big bang. Our surveys stride across the universe, swallowing millions of galaxies at a time. We have mapped and measured the most subtle accelerations of cosmic expansion.
But our understanding of all of that hinges on how well we know our own local neighborhood, which remains poorly mapped and poorly understood.
[Sign up for Today in Science, a free daily newsletter]
In April 2024 the Dark Energy Spectroscopic Instrument (DESI) collaboration made headlines with a stunning announcement. Data from the first year of a galaxy survey that captured precise measurements of more than 13 million galaxies revealed slight but significant evidence that dark energy may be weakening with time. That is, the mysterious force or substance that is causing the expansion of the universe to accelerate might be fading away.
This new result adds to the growing list of problems faced by the leading model of cosmology, called LCDM for “lambda cold dark matter,” which hypothesizes that around 95 percent of all the stuff in the cosmos is either dark energy or dark matter. For years cosmologists have struggled with the so-called Hubble tension, a discrepancy between measurements of the present-day expansion rate based on the nearby universe compared with extrapolations taken from the ancient, distant cosmos.
But to see out into the wider universe and make these grand measurements, we must first look through the nearby cosmos, and that may bias our observations. Like peering through a distorted lens, our perspective may give us the illusion of problems affecting the grand sweep of the heavens when really we’re just misinterpreting the data.
So what does our nearby universe look like?
The trouble with building a comprehensive map of the local cosmos (where “local” means out to a distance of a few hundred million light-years) is that to create a full picture, surveys must be deep, broad and complete. They need to map out every region of sky, going as far into the universe as possible, and capture every single galaxy, no matter how small and dim. Most astronomical surveys, however, typically achieve only two out of those three goals.
For example, in 2013 a team of researchers, Ryan Keenan, Amy Barger and Lennox Cowie, studied the possible existence of what was then known as the “local hole.” Since renamed the KBC void, it’s a possible depression in the local density of the universe stretching two billion light-years wide. It’s a not a particularly deep depression: this local span of the universe looks to be just 10 to 20 percent less dense than the cosmic average. But it might be enough to mess with our observations of cosmic expansion: the galaxies inside the void could experience an extra gravitational tug outward from all the higher-density regions on the outside, adding a bias that otherwise wouldn’t be there and potentially alleviating the Hubble tension.
Shortly after the DESI results, Indranil Banik of the University of Portsmouth in England and Vasileios Kalaitzidis of the University of St Andrews in Scotland invoked the KBC void to explain those findings as well, arguing that these measurements are distorted because they are, essentially, anchored on the wrong assumption.
Cosmologists assume, and comprehensive surveys have shown, that the universe is homogenous at large scales, meaning basically the same in all directions. A patch of sufficiently large volume should be largely like any other patch. Sure, there will be different arrangements of galaxies and clusters and voids, but the statistics of those structures—their sizes and separations, and so on—will be the same. Yet it’s not exactly clear where homogeneity kicks in or how far out our local universe can be different without running afoul of LCDM. Once we allow for the existence of the KBC void, Banik and Kalaitzidis argue, then the need for evolving dark energy might go away.
But we’re not exactly sure if the KBC void actually exists. It does appear, given our limited surveys of the nearby universe, that there are indeed fewer galaxies in our billion-light-year patch than there are outside of it. But again, comprehensive astronomical surveys are notoriously tricky.
In May 2025 a team of cosmologists published the findings of a multiyear study into the structure of the nearby universe that casts some doubt on the empty spot. The researchers’ tool wasn’t a new telescope or instrument, though. It was a computer.
The team took existing catalogs of galaxy surveys and fed the data into a computer simulation of the growth of cosmic structure structures and the appearance of galaxies in an attempt to create a complete picture of the local universe. But these simulations contain all manner of tunable variables, such as the amount of dark energy in the universe and the efficiency of galaxies’ star production. And the surveys themselves are far from complete, with gaps, holes and missing data; an unknown number of galaxies are too small and dim to be captured.
So the researchers used Bayesian statistics, a kind of statistical approach that cleanly incorporates prior knowledge and assumptions. They ran simulation after simulation, varying every possible parameter, to build up a suite of mock galaxy surveys that were statistically compatible with the actual data.
At the end of their exhaustive analysis, they were able to find matches to most of the known superclusters near the Milky Way with high statistical likelihood, implying that the clusters astronomers think they see in galaxy surveys are indeed the real deal and not a fluke of poor-quality data.
But there was no sign of the KBC void.
This study is not the final word, just a statistical analysis of the likelihood of the KBC void actually existing given the quality of current galaxy surveys. Plus, cosmologists don’t know if large density variations in the distribution of galaxies around the Milky Way are enough to account for the Hubble tension and the DESI results.
But the lesson of these studies is clear. As we continue to scan the wider heavens and pierce deeply into the abyss in search of cosmic mysteries, we can’t ignore the portion of the nearby universe that we call home.
“Your diamonds are not in the far distant mountains or in yonder seas,” Conwell reportedly said. “They are in your own backyard, if you but dig for them.”