If you touch two CLEAN blocks of the same metal together in space, they weld!
Atoms in solid metals move a bit. Touch two clean surfaces together, and the atoms can't tell they're in different blocks so they become one group of atoms, ie ONE SOLID.
(Gif: https://www.homemadetools.net/forum/cold-welding-gif-56268)

The reason it doesn't happen on Earth when you put two of the same metals together is because of oxygen, which causes metal to rust. That oxide layer sits the two metal surfaces, so atoms in each block see a layer of different atoms, and know that's their "limit" for movement.

If you're wondering if this has affected space missions, it has! The Galileo space probe sent to Jupiter couldn't deploy its high gain antenna on the way to Jupiter because the metal rods that were to open up the "umbrella" got cold welded together!
Graphic: NASA/JPL

It's not normally a problem because spacecraft parts are well lubricated and oiled, so there's always a layer between two metals of the same type. But Galileo waited in storage for nearly 5 years before being launched, so the lubrication probably wore off.

Another thing that prevents cold welding in space is using two different metals (then atoms can distinguish themselves from their neighbors because they're different!), plastic, or some other material, if it's to touch metal. This bypasses the need for a layer between two parts.

But HOW COOL IS IT that touching two surfaces of the same metal together in space can cause them to weld BECAUSE THE ATOMS CAN'T TELL THEY'RE PART OF A SEPARATE DISTINCT GROUP OF SOLID BLOCKS?! This is literally visible evidence of how the VERY SMALL works. PHYSICS IS PHENOMENAL

If you'd like to see me all mind blown and going nuts about this on video, check out my Instagram stories! https://www.instagram.com/astropartigirl/