다는

"the fact that…", before a noun

Packs a whole statement into a modifier for the noun that follows: 만들었다는 게 is "the fact that they made it", 솔로가 낫다는 생각 is "the thought that going solo is better", 남았다는 거지? is "so it means only they're left, right?". It is 다고 하는 ("which is said to be") with the middle squeezed out, so it can also report hearsay. In speech it turns up most often before the empty nouns 거 and 게, which is Korean's way of saying "that…" as a chunk. Its sister form -라는 does the same job when the reported piece is just a name or noun.