Within trillionths of a second after the Big Bang, they have reasoned, the primordial explosion - many billions of degrees hot - must have created a universe that contained equal quantities of matter and antimatter.
Scientists have reasoned that primordial explosion created a universe that contained matter and antimatter.
It's not clear from the context who are "they". It could be scientist but it could also be any other group. System assumes that it's scientists but that's not necessarily true or false.
( z1 reason :ARG0 ( z2 scientist ) :ARG1 ( z3 create :ARG0 ( z4 explode :mod ( z5 primordial ) ) :ARG1 ( z6 universe :ARG0-of ( z7 contain :ARG1 ( z8 and :op ( z9 matter ) :op ( z10 antimatter ) ) ) ) ) )