Direct messages on Lemmy (and most Fediverse platforms) are not end-to-end encrypted, which means that it’s stored in plain.

For example, your instance admin could theoretically see your private messages. I don’t think they check it tho, but if that’s a concern for you, then you should not share any sensitive details in your DMs. Also, don’t worry: random people cannot see it

Matrix is way better for that: it’s a decentralized communication platform, end-to-end encrypted, so only you and your recipient can read the messages.

I think it’s kinda the “Fediverse of messaging”