It is possible that Allah created the entire universe just for humanity; however, there are some hints that there are other creations out there and we simply don't know them. For instance the Qur'an speaks of the things that we do not know. A narration from Imam Ali (A) mentions that the other stars in the sky are cities or civilizations (mada'in) like the cities on earth. I think it is reasonable to assume that in the vast universe there are other beings, whether or not we will ever know of them during the span of existence of humanity, God knows best. However, what can be said is that what we have on earth is comfortable for us - especially if we consider the conditions of other planets like Jupiter - and that part of the miracle of creation is that we can look out at the sky and stars from our own vantage point and see it perfect for ourselves, in terms of timekeeping, light, heat, etc, and yet it can still host many other creations.