A theory explains abundance of hydrogen by reasoning that just after big bang, when the matter existed in form of sub-atomic particles. the hydrogen atoms were formed due to charge seperation between the particles. they reacted to form hydrogen molecules.
Although, we can't outrightly  say that its the most abundant in the universe because only a very small fraction of universe comes under observable universe and even out of this, only a very small fraction is observed and verified the abundance of hydrogen.
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