Shì wǒ gūlòu-guǎwén le, yuánlái zhège pǐnpái zhème yǒumíng.
I must be out of the loop — I didn't realize this brand was so famous.
Tips
usage
This idiom is almost always used as a humble self-deprecation: 我孤陋寡闻 ('I'm poorly informed') when admitting you didn't know something. It's rarely used to criticize others directly, as that would be very blunt.