While Reaper was looking for a way to contact Prime, Optimus was busy exploring the depths of space looking for what had (supposedly) killed Kronos. Around the time Reaper went looking for the old man, Optimus discovered an ancient temple at the heart of the universe. The temple dated back to just after the birth of the universe, making it as old as the titans. Such a thing should not be possible, since the universe the Titans had been created in was gone replaced by new planets, new stars, and new galaxies, as was the ever-changing nature of space. So the knowledge that something from basically the beginning of time was still intact, was a huge discovery. It also sparked a desire in Optimus's mind to explore the only thing left from the birth of the cosmos. And so he started exploring the structure bit by bit. He found a mural on the wall that was unfinished but told the story of the whole universe, from the start of it to the end of it. Optimus didn't have enough time to examine it though, because as he stepped closer to the mural, he activated a secret pressure plate, which opened up a staircase leading down.