What is 'that' referring to in the passage:

What is that referring back to in this paragraph ? American feminist activists who have been described as "solitary" and "individual theorists" were in reality connected to a movement―utopian socialism—which was already popularizing feminist ideas in Europe during the two decades that culminated in the first women's rights conference held at Seneca Falls, New York, in 1848.

[...] which was already popularizing feminist ideas in Europe during the two decades that culminated in the first women’s rights conference held at Seneca Falls.

“That” here refers to “the two decades”.

The two decades culminated in the first women’s rights conference held at Seneca Falls. “Culminate” means to reach a climax or point of highest development. In other words, the development of the first women’s rights conference reached a climax in the two decades.