Check out noun modifiers V+는/(으)ㄴ/(으)ㄹ and V+(았/었/였)던 and (았/었/했)을 것 for more info.
In casual or colloquial language,
Native speakers may colloquially write 는거 or 는것같다 (without the space). It doesn't strictly follow spacing rules, so its a common "mistake". The meaning is the same.