Final answer:
Gale should tell Stella that the far side of the Moon isn't always dark, we just never see it from Earth.
Step-by-step explanation:
Gale should say that the far side of the Moon isn't always dark; we just never see it from Earth. The Moon is tidally locked with Earth, which means the same side always faces us. This does not mean that the other side is always dark. In fact, sunlight does reach the far side of the Moon, just like it reaches the near side. However, since we cannot see the far side from Earth, it is often referred to as the 'dark side of the Moon'.
Learn more about The far side of the Moon and its visibility from Earth