How warm is it in the universe?

Answer

The temperature of the universe is 2,725 Kelvin, or about -270 degrees Celsius.
That temperature is that of the so-called cosmological background radiation.

Every body with a certain temperature emits electromagnetic radiation, and the wavelengths of that radiation are related to the temperature. For the sun (5800 Kelvin) that is the optical radiation that we see, we ourselves (about 300 K) radiate in the infrared. The universe originated as a hot big bang, and was therefore filled with high-energy radiation. Since then it has continued to expand, so the radiation has ‘cooled’ to the radio emission we observe today.

Since then, of course, new heat sources have appeared in the universe, and their surroundings are therefore warmer. The planets are heated by the sun. Why they are cooler the farther they are from the sun is simply because they can receive less sunlight further away: the light leaving the surface of the sun at any given time is spread out over an increasingly larger sphere, and the amount of energy per unit of surface area of ​​that sphere thus decreases accordingly. The temperature somewhere in the universe is therefore at least 2,725 K of the cosmological background radiation (there are some interesting exceptions), and is therefore higher if there is a star or other heat source in reasonable proximity.

Keep in mind that many (most, actually) places in the universe are very thin. Then it is sometimes difficult to speak of a unique temperature. Temperature is a kind of statistical concept, a measure of the average energy of a particle in an environment where particles exchange energy very quickly. In the interstellar center, this exchange takes place very slowly, and you often have a mixture of energetic (‘warm’) and slow (‘cold’) particles.