It's hard to be sure without more context. This is indirect, colorful, slightly joking language. A "dart" is literally a weapon. It's like a small arrow. It flies through the air, so it's fast. It surprises. It hits with force. To "dart in" means "to appear suddenly." He is describing a conversation. He is describing a sudden change in the topic of a conversation. It sounds like a serious emotional talk, as with a girlfriend. He was talking about wanting "more empathy." He is not quite sure how, but somehow a different topic "darted in." Without more of the conversation, we can't guess what he means by mathematics. Here's one way a phrase like this could appear: A: "I adore you. I'd like to spend my life with you. Marry me." B: "We can't afford to buy a house in Zenith on our combined salary." A: "I was talking about love, and somehow real estate sort of darted in." Here's another possible use. In a conversation, "to dart in" can mean "to interrupt." "The engineer was trying to explain why they needed an expensive lens, when the boss suddenly darted in with 'Yes, but how much is that going to cost us?'" "To dart" also means to make fast, sudden, unpredictable movements. " Raymond likes to stay in one lane when he drives. Lois darts in and out of traffic, trying to get ahead of the pack."