before one knows it; without noticing; unawares
Describes something happening while one is unaware or not paying attention; often used for gradual changes or unexpected events.
Before I knew it, it had started raining.
He left the room without my noticing.
A set phrase composed of 知らない (negative form of 知る, 'to know') + 間に ('while, during'). The literal meaning is 'while not knowing', which naturally extends to 'without realizing'.