〜もん・〜もの (excuses)
because, but (casual excuse)
Plain form + もん / もの · often with だって and んだ
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At the end of a casual sentence, もん (or もの) gives a reason as a self-justifying excuse: だって、知らなかったんだもん, "but I didn't know!"
It often pairs with だって ("but") at the start, and with んだ just before: 行きたくない。寒いんだもん, "I don't want to go. It's cold".
It sounds like a child or someone sulking, so it can be cute or whiny depending on the speaker. Adults use it jokingly among friends. もの is a little softer and more feminine than もん.
The polite relative is ものですから, "you see", from the start of this deck. Never use もん at work or with people above you.
Register: Casual, and a little childish or cute.
Examples
だって、知らなかったんだもん。
But I didn't know!
行きたくない。寒いもん。
I don't want to go. It's cold.
しょうがないよ、子どもだもん。
It can't be helped, they're only a kid.
だって、まだ眠いんだもの。
But I'm still sleepy.
食べられないよ、辛すぎるもん。
I can't eat it, it's way too spicy.