Answer:
2.73 Kelvin
Explanation:
At the beginning, the Universe was just a single hot, infinitely dense point of matter and energy, that later exploded and expanded over the time.
As the universe expanded over time, the average temperature of the universe decreased and decreased, until it reached the current value of 2.73 K.
This temperature corresponds to a peak wavelength in the microwave range: in fact, the accurate measurement of the temperature of the universe has been performed by measuring the Cosmic Microwave Background (CMB), whose distribution can be assimilated to the one of a blackbody radiator, whose corresponding temperature is 2.73 K.
