Tiny Part of Suntan is From Beyond Our Galaxy

                Not just the Sun, but the light travelling from galaxies across the universe for billions of years is also responsible for our suntans. We are constantly bombarded by about 10 billion photons per second from intergalactic space when we are outside, day and night. When we lie on the beach, our bodies are bombarded by about sextillion photons of light per second. Most of these photons, or small packets of energy, originate from the Sun but a very small fraction have travelled across the universe for billions of years before ending their existence when they collide with your skin. Astronomers measured the light hitting Earth from outside our galaxy over a very broad wavelength range. The study looked at photons whose wavelengths vary from a fraction of a micron (damaging) to millimeters (harmless).

                Radiation from outside the galaxy constitutes only ten trillionths of your suntan, so there is no immediate need for alarm. Most of the photons of light hitting us originate from the Sun, whether directly, scattered by the sky, or reflected off dust in the Solar System. However, we’re also bathed in radiation form beyond our galaxy, called the extragalactic background light. These photons are minted in the cores of stars in distant galaxies, and from matter as it spirals into super massive black holes.