15 billion years ago, the universe, as we know it, didn't exist. There were no stars, no galaxies, no atoms, no time, no life, no nothing. All that existed was infinite possibilities, and based on the fact that we are even having this conversation, nothing was off limits.
Understanding the moments that preceded the big bang is instrumental in understanding how things got to how they are now. Of course, we'll never truly know what came before. I mean, even if we had a time machine, the whole 'time not existing' thing would throw a wrench in our plans. However, we have a few ideas, which you can learn about below: