expression, noun
past events; bygones; water under the bridge
Refers to things that have already happened and are now in the past; often used in expressions about letting go or not dwelling on the past.
過ぎたことは仕方がない。
What's done is done; there's no use crying over spilled milk.
過ぎたことをいつまでも悔やんでも仕方がないよ。
There's no point in regretting the past forever.
過去 is a more formal, general noun for 'the past', while 過ぎたこと is a phrase emphasizing events that have passed and often carries a nuance of letting go.
Phrase composed of the past tense of 過ぎる (to pass) + こと (thing/matter), literally 'things that have passed'.