The average temperature of the universe is about 3 degrees above absolute zero. The absolute zero temperature is -273.15 degrees Celsius.
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To measure the temperature in deep space, there must be the presence of matter, since that is how we define temperature. The temperature of the room you are currently in is determined by the average movement of all the air molecules in the room. The more energy they have, the faster they move and the higher the temperature. For example, if you touch a very hot object, its atom and molecules vibrate violently, giving it a very high temperature.
There is not much matter in interstellar space. The average density of the universe is only about one hydrogen atom per cubic meter. This makes it very difficult to determine the temperature of matter in interstellar space. But space itself is saturated with an endless sea of radiation, which is very, very cold.
This radiation comes from stars, galaxies and more, but by far the biggest source of radiation in the universe is the cosmic microwave background (or CMB).
The CMB appeared when the universe was about 380,000 years old. At that time, our cosmos was about a million times smaller than it is today, and it was in a state of hot, dense plasma. As the universe expanded and cooled, it became neutral, giving off radiation that was about 10,000 Kelvin, which is the temperature of the Sun’s surface.
This radiation accounts for more than 99.999% of all radiation remaining in space. Since its inception, the Universe has expanded, which led to the weakening of the same radiation, lowering its temperature. In addition, the cosmic expansion is spreading into the light itself, moving it to longer and cooler wavelengths.
The combined action of this expansion lowered the temperature of the CMB by about 3 degrees above absolute zero. This means that if you were in interstellar space, your body would cool down to absolute zero. But it would not allow reaching such a temperature, because the cosmic microwave background radiation will always affect you, transferring its energy into your body. So you wouldn’t reach absolute zero, but you would come into equilibrium with the CMB, which is how scientists define the (cold) temperature of interstellar space.
