Lying in the vicinity of the Bo\u00f6tes constellation, it became known as the Bo\u00f6tes Void, or sometimes the Great Nothing. For a long time, it was the largest known void in the universe, spanning 330 million light-years<\/a> across. To put that in context, that’s about 0.35 percent of the diameter<\/a> of the entire observable universe.<\/p>\n A big span of nothing.<\/p>\n While it is fair to describe it as a void, there are galaxies within it, just a lot fewer than we would expect.\u00a0<\/p>\n “If we are to use a rough estimate of about one galaxy every 10 million light-years (four times farther than Andromeda),” NASA explains<\/a>, “there should be approximately 2,000 galaxies in the Bo\u00f6tes Void.”<\/p>\n In fact, we’ve found 60. While there is little about the void to suggest our ideas about galaxy formation<\/a> are incorrect \u2013 one explanation is that it formed from smaller voids merging \u2013 it is still an odd thought experiment to picture how someone inside the void must see the universe.\u00a0<\/p>\n As astronomer Greg Aldering put it: “If the Milky Way had been in the center of the Bo\u00f6tes void, we wouldn\u2019t have known there were other galaxies until the 1960s.\u201d<\/p>\n That was\u00a0the largest void that we know of. But in 2015, a team found evidence<\/a> of a much larger void, measuring a whopping 1.8 billion light-years across. The team were looking in the direction of the infamous cold spot<\/a> of the cosmic microwave background<\/a> radiation, believing that the cold spot could correspond to a massive void.<\/p>\n Sure enough, they found evidence<\/a> of a void spanning 1.91 percent of the diameter of observable universe, although the void is still not large enough<\/a> to explain the mysterious cosmic microwave background (CMB) cold spot. The KBC Void, with a radius of around 1 billion light-years<\/a>, is notable for containing our home galaxy, the Milky Way.<\/p>\n Like the Bo\u00f6tes, the KBC Void (or Local Hole) is not a true void, but an area of underdensity, with measurements varying from around 15-50 percent less dense than the surrounding regions. Merely counting the number of galaxies supports the idea of a local void, as there are fewer galaxies in our local region than in the neighboring regions.<\/p>\n There are quite a few reasons why this may be interesting. For one, our models predict that matter should be distributed fairly evenly through the universe at large scales, and this is at odds with those predictions. But where it gets really interesting is when looking at the expansion of the universe, and the “Hubble tension”.<\/p>\n \u00a0\u00a0\u00a0<\/p>\n The Hubble tension<\/a>, for the uninitiated, refers to the fact that measurements of the expansion of the universe differ depending on how you measure it. Looking at the CMB radiation \u2013 the first light of the universe from around 400,000 years after the Big Bang \u2013 puts the rate of expansion at 67.4 kilometers per second per megaparsec, with 1 megaparsec being 3.26 million light-years. Meanwhile, measurements of how fast objects move away from each other in the local universe \u2013 focusing on “standard candles” like type 1a supernovae and Cepheid variables<\/a> \u2013 puts the expansion rate at 73 kilometers per second<\/a> per megaparsec.<\/p>\n<\/p>\n