Science

This is the coldest point of our entire galaxy (and maybe the entire universe). 
It is at the INFN (National Institute of Nuclear Physics) in Italy. 
It’s just 6 millikevin (-273.144 C)

How do we know there's no colder point in the infinity of the universe?

The background radiation of the universe is ~2.5 kelvin

And nothing can be colder than the background?

Even in places that we don't understand like black holes or dark matter or anything unknown? I mean, it seems strange to me to say it's impossible when we know we know nothing...

Nothing natural. Even the interstellar void cannot go below the background radiation. 

Maybe on another planet with aliens that can manage technology like us, there may be another detector like that.