The Absolute Emptiness
In high vacuum systems, pressures of 10-6 / 10-8 mbar are normally reached, i.e. from one billionth to one hundred billionth of atmospheric pressure. In the small UHV chambers available in research laboratories, the degree of vacuum that can be reached through the use of special pumping systems can even reach 10-12 mbar, which is a million times better than that used in normal industrial systems.
How many particles are present then, under these conditions, in one cm3 of volume, at room temperature, that is - let's say - 20 ° C? We can consider the so-called molar volume, i.e. the volume occupied by a mole (i.e. 6.02x1023) of particles: in the indicated conditions and considering a perfect gas - the gases composing the air, in the vacuum conditions of our interest, behave more or less as such - it is worth a little more than 22 liters, or 22x103 cm3. Therefore, at atmospheric pressure, there will be 2.7x1019 particles in one cm3 of volume. At the (more or less) minimum pressure that can be reached with high-level equipment in our terrestrial laboratories, in one cm3 of volume there will be 2.7x107 particles, or almost 30 million particles of gas!
In space, things are a little better.
For example, the density of particles in the space inside our Solar System is about five atoms per cm3. In interstellar space, inside our galaxy, this concentration drops only one atom per cm3, while in intergalactic space the presence of only 0.01 atoms per cm3 is estimated (which in any case is 10,000 atoms per cubic meter...).
Is absolute emptiness therefore reachable? The answer is no, as quantum mechanics dictates the fact that, even in "empty" space, the inevitable energy fluctuations present determine the appearance and disappearance of so-called "virtual particles".