The matter that gets pushed beyond the observable universe due to the expansion of space is not necessarily "destroyed" in the sense of being annihilated or ceasing to exist. It is important to understand that when we talk about the observable universe, we are referring to the portion of the universe that we can currently observe given the limitations of our instruments and the finite speed of light.
The expansion of the universe means that the space between galaxies is increasing over time. This expansion can cause galaxies to move away from each other at speeds greater than the speed of light, which means that their light will no longer be able to reach us. As a result, these galaxies will eventually be outside the boundary of our observable universe.
However, it is important to note that the expansion of space does not directly affect the matter within galaxies or other structures. It is the space itself that is expanding. The matter in those regions is still subject to the gravitational forces that hold it together, and local interactions between galaxies and other celestial objects can continue to occur.
So, while matter may move beyond the boundary of our observable universe, it does not necessarily imply that the matter itself is destroyed. It is simply that we can no longer observe or interact with it directly due to the expansion of space.