Roughly how many galaxies are in the observable universe?

Answer

Older estimates said 100 to 200 billion. Deep Hubble images and modelling raised the estimate, which remains uncertain.

Context

For decades, galaxy counts relied on the Hubble Deep Field images, which suggested about 100 to 200 billion galaxies in the observable universe. In 2016 a team led by Christopher Conselice reanalysed deep Hubble data, combined with other surveys, and converted the images into three-dimensional maps of galaxy numbers across cosmic time. They concluded the true number is at least ten times larger, around 2 trillion.

The reasoning rests on the early universe. About ten times as many galaxies filled a given volume long ago as do today, and most were small and faint, similar in mass to the Milky Way's satellite galaxies. They merged over billions of years into the larger galaxies we see now. Models in the study implied that around 90% of galaxies are too faint and distant for current telescopes to detect, so most of the 2 trillion are inferred, not observed.

The result is an estimate with uncertainty, and it depends on how many faint dwarf galaxies exist, which is hard to measure. Later work with the James Webb Space Telescope has found many more faint galaxies at early times, but a precisely agreed total does not exist. The 'observable' qualifier also matters: it refers only to the region from which light has had time to reach us, about 46 billion light-years in radius, and says nothing about the size of the whole universe, which may be far larger or infinite.