Why is true vacuum so hard to create? What exactly makes the matter “want” to go into a empty space? What creates that pressure?
Imagine air atoms are like floating balls moving freely and constantly bumping into each other.
Suppose we have a container and we want to remove all air from it, what do we do?
We could poke a hole into this container and leave it. Over time air atoms inside the container will keep moving inside the container until they eventually - one by one - exit the hole. Problem with that approach is that there is also air atoms outside that would also enter the hole and you end up with roughly same concentration of air atoms inside and outside.
But lets say we somehow took that box and placed it into a perfect vacuum. Now air atoms inside the box will go outside but nothing from outside will go back to the box, that should make the box have perfect vacuum right?
Unfortunately not.
After the vast majority of atoms have exited the box, only very few atoms remain and they hardly collide with eachother anymore. Then their movement can be stabilized. For example one atom would keep moving in horizontal or vertical motion indefinitely and never ever get close to the hole. With no atoms to collide with it and change its path, it may never exist the box. That’s why its almost impossible to have perfect vacuum