Simplify the following text for a Grade 15 native English-speaking person.
Original: The relative fractions of different chemical elements â '' particularly the lightest atoms such as hydrogen , deuterium and helium â '' seem to be identical throughout the universe and throughout its observable history .
Simplified: The relative amounts of different chemical elements â '' especially the lightest atoms such as hydrogen , deuterium and helium â '' seem to be identical in all of the universe and throughout all of the history of it that we know of .