If we pointed our telescopes at a galaxy 1 million light years away and that galaxy had a giant mirror pointing back at us, we would see our galaxy as it was 1 million years ago. This is because the light we receive from distant objects, such as galaxies, takes time to travel to us. When we observe objects in space, we are essentially observing them as they were in the past because the light we receive has traveled across vast distances before reaching our telescopes.
In this scenario, the light from our galaxy would travel from our galaxy to the distant galaxy's mirror, then reflect back towards us. The light would take 1 million years to reach the distant galaxy and another 1 million years to return to us. Therefore, the image we would see in our telescopes would depict our galaxy as it appeared 1 million years ago, capturing the state of our galaxy at that time.
It's important to note that this hypothetical scenario assumes the existence of a giant mirror capable of reflecting light across such vast distances, which is currently beyond our technological capabilities. Nonetheless, it helps illustrate the concept of observing distant objects in space and the time it takes for light to travel.