A very small pop - possibly the worlds smallest thunderclap
Surprisingly the average thermal velocity of air molecules (or any ideal gas) is around the speed of sound at that temperature and pressure.
This is about 330m/s at room conditions so the air would rush to fill the 0.3m gap in 1ms
Ultra High Vacuum goes down to pressures of perhaps $10^{-12}$mbar, which corresponds to a particle density of some $10^4$ molecules per cm$^3$. The interstellar medium has densities of perhaps 1 particle per cm$^3$ (Wikipedia agrees). So indeed the number density of particles in outer space can be much lower than that of a very good vacuum in a lab. Unless you are in a molecular cloud or such, in which case it might as well be much higher of course.
The mass-energy density of dark energy is tiny in comparison to, well, in comparison to anything really: About $10^{-29}$g/cm$^3$ which is another factor $10^6$ or $10^7$ lower than the density number of particles in interstellar space quoted above.
Any quantum fluctuations in the vacuum of a mercury barometer are utterly negligible. One can however measure the Casimir effect in the lab, but this is the infamously "worst model in physics" for dark energy, so again, no information about dark energy from that at all.
So then there is the question whether the vacuum chamber in your lab would contain dark energy or not. Since dark energy is believed to be a property of spacetime itself, the answer is expected to be "yes". So, no, you not would expect the vacuum we create to be "purer" than the vacuum in outer space. However, given the above order-of-magnitudes, it should be clear why there can be no experimental evidence to support that.
Finally, you mention dark matter. Dark matter interacts only very weakly, so we would expect that it simply passes through any vacuum vessel you build. In other words, just like with dark energy but for other reasons, the density of dark matter is also expected to the same, whether you are in outer space, in your lab breathing air, or in some vacuum vessel. Specifically, that density is of order $10^{-25}$g/cm$^3$ in the vicinity of our Sun.
Best Answer
Yes, in space this would be possible.
The reason that one cannot open up a space inside a plastic bag when it is empty and sealed is because of the air pressure surrounding the bag. A huge number of air molecules are constantly bombarding the plastic bag from all directions (a.k.a. air pressure), so it would take a large amount of evenly distributed force to open the bag despite the pressure. Pulling on it with your fingers doesn't cut it.
If you were in the near vacuum of space, that air pressure would be almost entirely gone, so I predict that you could indeed fairly easily open up a space inside a sealed, empty plastic bag (neglecting the fact that current space suits have very clumsy fingers that might not be up to the task).