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You fill all earths emptiness
The higher I go, you get less
If you go away nobody will stay
The jellyfish might still play
Oh I, representative of all being
Oh I, just you I will keep needing
Oh you, not visible but irreplaceable
Oh you, so essential but untraceable
I guess the "you" in the riddle must be
Air
You fill all earths emptiness
The atmosphere of Earth is the layer of gases, commonly known as air, that surrounds the planet Earth.
The higher I go, you get less
The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space.
If you go away nobody will stay
Most animals need oxygen to survive.
The jellyfish might still play.
Jellyfish can absorb the oxygen in water although there is an animal that looks a bit like a jellyfish and can survive without oxygen.
Oh I, representative of all being
The "I" in the riddle must represent life in general or possibly the life that exists on land.
Oh I, just you I will keep needing
All land animals need air to survive
Oh you, not visible but irreplaceable
Air is invisible.
Oh you, so essential but untraceable
The one shortcoming of this answer is that air is not entirely untraceable but at least hard to track and predict.
