Hello again and thank you for all your kind feedback! I think you helped me solve this mystery.
You guys asked about moist. No, there was no moist from the microwave oven, of that I am sure. The vent was always turned away from the wall, it was never hot or gave away water. And the reason I know the cavity was not always there is because we refurnished this kitchen years ago and set up the counter, the microwave and everything else. We would surely see the cavity rubble on the floor (the fourth photo) and clean it up. So it all had to fall down after it was all set up.
So where is the culprit? I posted another photo – under the counter and the microwave, in the red circle was a refrigerator. It stood there as many years as the microwave and it did give off moist. And it’s likely that this moist went by the side of the counter, up the wall and through the years dug the cavity.
That I think is the most likely what happened.
Thank you for all of your answers!