Where does the "적이" come from?
한 적이 있다/없다 is a grammar form for expressing if one has ever done something before.
오페라를 본 적이 없어? - You've never seen an opera?
Would be nice if DLG would add some reference note to the translation above. e.g.
[V] + (으)ㄴ 적이 있다/없다
[Have ever (V) before / Have never (V) before]