The sun warms up our planet but space is cold. If I lit a candle and stood 20 feet away, I could see the light but not feel the warmth. So, why do we feel warmth from the sun but space isn’t warmed by it?
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A lot of other commenters have pointed out that there’s a misunderstanding of what we mean by “hot” when there’s nothing to heat up. But, it might interest you to know that while space is effectively empty compared to the earth (10^24 atoms/cm3) and its atmosphere (10^21 atoms/cm3), it’s not literally empty. There are still particles around in the solar system (5-40 atoms/cm3), and continuing downward as you leave the galaxy (10^-3 atoms/cm3).
These particles, however few they are, actually are quite hot (according to the physics definition, which is that temperature = average speed of particles), and astronomers can generally show they can be anywhere from 10^4 to 10^7 K — much hotter than air is on Earth! It gets heated up by stars and gravitational acceleration, and cools quite slowly.
It doesn’t feel hot when you’re there, because only a few particles even hit you per second, but each one will slightly heat you up! You’re just so complex chemically that it’s much easier for you to cool back down by radiation than it is for the particles, so they would never heat you up to their average temperature, no matter how long you waited.
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