Simplify the following text for a Grade 11 native English-speaking person.
Original: The heliosphere is a bubble in space `` blown '' into the interstellar medium ( the hydrogen and helium gas that permeates the galaxy ) by the solar wind .
Simplified: The heliosphere is a bubble in space `` blown '' into the interstellar medium ( the hydrogen and helium gas that fill the galaxy ) by the solar wind .