The average integrated light per unit volume of the Universe, which provides a useful constraint on the overall star-formation history since the Big Bang. The present-day luminosity density in optical light of the local Universe (and hence, according to the cosmological principle, the entire Universe) is around 50 million solar luminosities per cubic megaparsec. In other words, an average cube one million parsecs on a side contains the light of 50 million Suns. For comparison, a typical galaxy is around 10 000 parsecs in radius and contains a billion or more stars; hence most of space is devoid of stars.