Pressure is directly proportional to particle number density. Under standard conditions, the number of particles per unit volume is extremely large; even at a pressure of 10-12 hPa, there are still 26,500 molecules per cubic centimeter. This is why an absolute vacuum cannot be said to be achieved, because molecules exist even under ultra-high vacuum.
In space, it gradually becomes difficult to express extremely low pressures in pascals, for example, below 10-18 hPa. These pressure ranges can be better represented by particle number density; for example, in interstellar space, the number of molecules per cubic meter is less than 104.
