The observable Universe contains about two trillion galaxies—more than ten times as many as previously estimated, according to the first significant revision of the count in two decades.
Since the mid-1990s, the working estimate for the number of galaxies in the Universe has been around 120 billion. That number was based largely on a 1996 study called Hubble Deep Field. Researchers pointed the Hubble Space Telescope at a small region of space for a total of ten days so that the long exposures would reveal extremely faint objects.
This view encompassed galaxies up to 12 billion light years away, which we see as they existed less than two billion years after the Big Bang. Astrophysicists then counted the galaxies within that narrow field of view and extrapolated the number to the full sky—under the assumption that it would look similar in all directions—to get to the 120 billion figure.
However, there weren't enough galaxies in the Hubble Deep Field image to account for the density of matter distributed throughout the Universe. The missing matter had to be in the form of galaxies too faint to see, as gas and dark matter. “We always knew there were going to be more galaxies than that,” says astrophysicist Christopher Conselice of the University of Nottingham, UK. “But we didn't know how many existed because we couldn't image them.”
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