What Limits the Space from being empty?  

To get to the answer, we will have to travel back in time and understand the background story that led physicists to think of space that way. In the mid-1900, Stephen Hawking proposed his theory about black holes, known as the second law of black hole dynamics. It says that" the black hole can get larger. The size of a black hole is determined by its mass, a black hole gets larger anytime anything new falls in and adds to the mass. If nothing can get out of a black hole, its mass cannot possibly decrease. A black hole cannot get smaller".  

This discovery of Hawking has a similar ring to it, the second law of thermodynamics. This law states that" the disorder always increases, it never decreases". This resemblance between the two laws made physicists think about their mutual relationship. A graduate physics student at Princeton, Jacob Bekenstein, came forward to unravel the relationship. He insisted that the second law of black hole dynamics not only resembles entropy, but it is also entropy.  

This explanation of Jacob Bekenstein gave birth to an even more bewildering question. If something has entropy, it has temperature, if something has a temperature, it must radiate energy. That means black holes must also radiate energy if Bekenstein is right. Hawking was irritated by what he thought was Bekenstein's misuse of his discovery that the event horizon never decreases.  

Hawking decided to tackle this issue through quantum mechanics. The use of quantum mechanics produced far more dramatic results; black holes should radiate energy. This proved that Bekenstein was right. But Hawking was into a puzzle now. How can a black hole possibly have a temperature and emit particles if nothing can escape past the event horizon? He found the answer in quantum mechanics.  

Spacetime fabric

Now we are in a position to answer the question.

Hawking defines uncertainty principle uniquely-We can never know both the value of a field and the rate at which the field is changing over time with complete accuracy- The more precisely we know the value of a field, the less precisely we know the rate of change. That means the field cannot be exactly zero. Zero would be a very precise measurement of both the value and the rate of change of a field. And uncertainty principle would not allow that. You do not have empty space unless all fields are exactly zero: no zero, no empty space. Then what do we have instead of empty space?  

Instead of empty space, we have the value of the field wobbling towards the positive and negative sides of a zero, so as never to be exactly zero. This fluctuation in the field is due to the production and annihilation of pair of virtual particles in space. Quantum mechanics tells us that this is happening all the time, everywhere in the vacuum of space. Even if they are only virtual particles, we know they exist because we can measure their effects on other particles.  

A very well-established principle was uniquely defined to answer a strenuous question. How many more golden answers does the uncertainty principle possess? It only requires someone to define it idiosyncratically.