Museums generally keep moon rocks under vacuum. Let’s consider the difference in atmospheric pressure on the earth (760 torr) and the moon (10-12 torr). The pressure on the moon is actually close to the lowest we can generate on Earth, and it’s closer to the vacuum of outer space (10-17 torr) than our pressure. If you open the chamber, air would rush in quickly and press on the rock surface. Thankfully, it wouldn’t implode because rocks (including ones from the moon) can’t be compressed under pressure. This means they don’t change volume when exposed to high or low pressures, the way gases do. (Liquids are hardly compressible either.)

The rock could be broken with enough applied force (like if we focused that blast of air on just a small section) but if we expose the whole moon rock to air and do it slowly, the rock won’t break. Generally, explosions or implosions only occur when the volume of a gas changes (due to change in pressure or temperature) inside a container than can be deformed under the force. Since the rock itself can’t be deformed, even gas pockets inside the rock (which are unlikely) wouldn’t feel change in pressure.

So, if it’s not to keep the moon rock together, why keep it under vacuum? It’s to keep water, dust and microbes from Earth out. Studying moon rocks gives us insight into how the solar system and the Earth were made, and they were very difficult to obtain. Even the rocks you see in the museum are still being studied and tested to learn even more. By sucking out all the air in the chamber, we also reduce the risk of contamination, so that the rock you see is exactly the same as when it was taken off the moon.

As we think about this scenario, let's imagine what happens in each step of the process.

We should first notice this; if the rock was found at the moon, we would have to put this moon rock into a vacuum chamber before transporting it. This would result in a rapid decrease in pressure, going from moon pressure to 0 pressure.

Next we look at the ideal gas law (pressure and volume are proportional to the number of moles and the temperature). When all other parameters were held constant, the pressure change must result in some opposing volume change. If you think of this with a marshmallow, when it is at atmospheric pressure the marshmallow is fluffy and light. If you drop the pressure putting the same marshmallow in a vacuum, you'll see it increase in size. If it were a perfect vacuum, the 'mallow would take up the whole space of the container!

Finally, we have our moon rock and we bring it back to Earth. Now we are at normal atmospheric pressure, which is relatively high. We take our moon rock out of vacuum, where its pressure increases drastically. Since pressure is caused by force over an area, the pressure does affect the rock. Appropriate force is applied to the rock from all sides when the pressure is very great. However, (again) due to inter-molecular forces in the solid, the pressure change would not make any observable difference. In cases in which the pressure is infinitely big, there is a good chance the rock experiences an implosion when taken out of vacuum, as it exceeds these forces within the rock.

In the same process, there is a way to turn the rock from a solid to a liquid and/or gas solely by changing the pressure. If at the right temperatures with the right materials, (CO₂ is seen to do this) one could drop the pressure low enough to change the phase of the "moon rock" into either a liquid or gas. The material then loses it's strong inter-molecular forces and is free to follow the imposed change in volume, as seen by the ideal gas law. In putting it in a vacuum, the pressure change could transform this rock into a liquid or gas, however, assuming the moon rock is made of some mixture of earthly metals (like silicon, iron and magnesium), this most likely will not occur.

In this case, if we take the liquid/gas material in the vacuum chamber and slowly increase the pressure to atmospheric, we can see the object condense into a similar, solid material seen on the moon.

I hope this answers your question!

Neither. If the moon rock is really a rock, then it is just as solid as a rock on Earth would be (and, specifically, a kind of lava called basalt, which most of the moon's surface is made out of). Much of the rock on the moon formed from molten lava, something which is impossible without pressure to keep it from boiling. This suggests that the moon might have had an atmosphere at one point!

The moon rock just sits on earth looking boring.