[svachalek]

It is believed to be the cooling of the universe. At ridiculously high temperatures, such that have not existed since the first fraction of a second of the universe, the electroweak symmetry was broken and most physics we are familiar with didn't work. Unfortunately the math behind it is way over my head so that's about all I can say on it.

[benreesman]

Not a physicist, just a fan. As far as I understand it, we believe that it was in the early universe that the symmetry was unbroken [1].

[1] https://en.wikipedia.org/wiki/Electroweak_interaction#After_...