the function of that [closed]

In the sentence below:

Please tell me what does "that" function as here. I do not think it is a relative pronoun or something similar.

What they proclaimed, these posters, was the existence of another world, of such modernity, such intensified energy and speed, of danger too, that their local one of weatherboard houses and bakers’ carts, unweeded pavements and trams that filled the night sky with electric sparks, seemed by comparison flimsy and becalmed.

"That" is subordinator functioning as a 'marker' of the declarative content clause in bold.

The content clause is called an indirect complement because although it follows the coordination of noun phrases "such modernity, such intensified energy and speed, of danger too" it is not this noun phrase that licenses it, but the two occurrences of "such" that modify the noun phrases.

In other words, "such" and "that" are in construction together. We can tell this because if "such" is dropped, the clause becomes ungrammatical.