"The way Gaster ended up in the void was through a experiment with a machine, the purpose of said experiment was to get a better understanding of space-time."

Under this theory, Gaster previously studied determination in search of a method to break the barrier, but he did not advance as much in that topic since his studies began to be oriented towards the understanding of space-time, as well as timelines. This was because during the first months of research, a power was discovered in the souls, whose potential would possibly be related to space-time, and it would be prudent to expand that field before continuing with the investigation of determination (Which Alphys never did because she ended up injecting some monsters with it directly, basically took another approach).

To speed up the process, Gaster came with a group of scientists friends, who are the current known Gaster's followers, and Sans as a temporary assistant due to his sentinel rotation. Sans wasn't really a scientist, he was just there for the food. Various experiments were performed, including the creation of the Gaster Blasters as an attempt to continue the determination experiments, but as they were not their priority, the original blueprint was not realized, and instead they were reduced to being a mere weapon that was granted to the sentinel to improve efficiency in its work of capturing humans.

One fateful day, the ambitions of the experiment with a machine caused a disaster, and all previous entries reporting its development were destroyed, except for a fan drawing made the day before the "Great Experiment" that they did not were going to forget. The scientists were completely missing, and the laboratory was destroyed, so it was assumed that the "explosion" (which was actually an implosion) had finished them off.

However, these had not been completely destroyed, but ended up in a place known as the void. In this void they would meet GonerKid and ClamGoner. These characters were already in this place, at the game level they were test subjects, simple "vessels" that served for the purposes of game programming that later remained in the code. From this void, Gaster would try to carry out some experiments to find a way to return, but it was difficult for a long time and the insanity of the place began to corrupt him little by little. The only entry he would write in that place is 17.

Sans from his part was not affected by any of this because he had fallen asleep in his guard post, so he did not attend the laboratory that day (and almost none actually because he was so lazy). Reports of an explosion sound coming from the laboratory made him have to go there for his role as a sentinel, and upon finding that scenario, he would inform the King and he would be in charge of safeguarding the machines and unfinished plans to avoid future problems. Papyrus never found out anything.

Some time later Alphys would be hired as a new royal scientist.