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.
The word “that” can be used in various ways (see Lexico). Two of them are:
2 Referring to a specific thing previously mentioned, known, or understood.
‘that’s a good idea’
5 relative pronoun
Used instead of ‘which’, ‘who’, ‘whom’, or ‘when’ to introduce a defining clause, especially one essential to identification.
‘the woman that owns the place’
In your quote, if “that” referred back to “utopian socialism”, the author would be asserting that the culmination of utopian socialism lasted for the 2 decades associated with “the first women’s rights conference …”. Although you can argue the ‘that = demonstrative’ case syntactically, it is more natural to treat the word that as a relative pronoun here.