Whenever I try to ponder eternity, my mind rebels and lets me know it can't comprehend that idea.
However, the Prophet Joseph Smith taught: "That which has a beginning will surely have an end, take a ring, it is without beginning or end -- cut it for a beginning place and at the same time you have made an ending place ... so it is with God." ("Teachings of the Prophet Joseph Smith, page 181.)
Thus, there has always been a God.
Our God, the Father, has his own Father and so forth backward into eternity. There is a plurality of endless Gods.
There's no "Big bang" needed here -- that's man's cop out for understanding.
Somehow, a "God" has always existed. If not, there will be an end.
The universe has always been here in some degree.
Joseph Smith also stated that "God himself could not create himself." And, he stated that the spirit of man has always existed.
(Joseph Smith Teachings, page 354.)
Somehow God can stop light and create matter. Since he can likely also produce endless light, there's endless matter around. He just adds "intelligences," when needed to the mix, to create life, though there may be life to some degree in all matter.
This concept of "no beginning" is what makes my head spin and even rebel at the concept ... it is currently beyond my understanding.