When I read about how Hashem created the universe and all living and non-living things in it, I often ponder on the need for creating the universe in all its detail. Why was it necessary to create so many billions of stars and planets or to create millions of live species? Would the world have been any less glorious if there were a few less species? Or a few less stars and galaxies? Why was this amount of vastness and detail required?

Answer: 

Very good question indeed. 

That is if you can claim to have researched every element of existence and coexistence and came to substantial conclusions that certain things are unnecessary for the greater benefit of the world.

Until then it is premature for us to say "this is not needed" or "why bother creating this." How can we possibly rule something out just because we haven't bothered or weren't able to understand what it does.

It is said about King David that he once told G-d, "I understand why you have created everything in this world except for spiders and mentally challenged people; why on earth did you create those two?"

It is safe to assume that just as no engineer puts unnecessary parts into the space shuttle, so too does G-d not put any unnecessary elements into His creation

It happened once that David (not yet king) was running away from King Saul's henchmen who wished to kill him. Tired and with the enemies at his heels David climbed into a cave. Moments later the King's henchmen came to the cave and thought of searching it, but they noticed a spider web across the entrance of the cave and figured no one could have entered the cave without breaking the web. In reality, a spider came and rapidly spun a web shortly after David entered the cave.

It happened again some years later that King Saul wanted to kill David, and David had to flee Israel. He ended up in the land of one of the Philistine kings. The Philistines were sworn enemies of the Jews and soon rumor spread that David, the same David who slew the Philistine super-hero Goliath, was actually in their land. Word reached the king and soldiers were sent to find David.

Meanwhile, David also heard these rumors and he was petrified. He came up with a planned disguise; he acted as if he were mentally retarded. He walked around drooling and writing on the walls that the king owed him money. The charade worked like a charm.

So until we can understand every mystery in creation, that is if we could ever understand, it is safe to assume that just as no engineer puts unnecessary parts into the space shuttle, so too does G-d not put any unnecessary elements into His creation.

Every single creation in this world can be used for some G-dly purpose. G-d created you, the intelligent human, so you would figure out how.