Board Thread:Questions and Answers/@comment-98.237.16.119-20151023070346/@comment-2112031-20160423142557

It's already been stated that, in the original timeline, Harrison Wells and his wife, Tess Morgan, built and activated the particle accelerator in the 2020, which is when Barry got his powers.

Eobard Thawne changed the timeline by killing Nora, Harrison and Tess and then impersonating Wells in order to build the particle accelerator faster.

I'm guessing that in the original timeline, when Eobard first traveled to 2016 and caused a truck to lose control, Barry didn't arrive to stop the truck (as he wasn't the Flash yet) and Eobard presumably traveled further in time in order to find the time period in which Flash exists, which we know he eventually did.