Can someone DM you if you don't follow them back?

The fact that they didn't follow you tells you very little on its own. Plenty of people only follow accounts they check regularly, and some avoid the follow button entirely because they don't want their follow list to be public or they think it looks like a commitment. What the message content tells you is much more useful. If they reference a specific post, a photo, or something you wrote, they paid attention, and the reason they wrote is probably that thing. If the message is generic, like hey or hi, they are either testing the waters or sending it to many people, and you should read it as lower effort.

Look at their profile before replying. You don't need to dig deep. Check whether the account is active, whether it looks like a real person, and whether what they said matches what they show. This is about making sure the conversation is with someone real, not about deciding whether they deserve a reply. Most of the time the profile will show someone ordinary who wants to talk.

The best replies in this situation are the ones that answer their question or respond directly to the thing they noticed. If they asked where you went hiking, tell them. If they complimented a photo, say thanks and add one real detail about it. The reply should be short enough to feel easy to answer and specific enough that they know you read it. Two sentences is often plenty.

What the reply should leave out is anything that tries to reframe the message. Don't ask why they didn't follow you, don't joke about them sliding into your DMs, and don't close the door by sending only a yes or no with nothing else in it. A reply that gives them something to respond to is what keeps the conversation going. A reply that makes them defend their message usually ends it.

Reply within a day if you can, and during normal waking hours for both of you. A message sent late at night often reads as more intense than it was meant to be, and a late reply can feel like you forgot about it, even if you didn't. If you are busy, a quick acknowledgement and a fuller answer later is fine.

If they don't answer after your reply, wait several days before sending anything else, and then send only one light follow up that relates to the first conversation, such as a question about the trip or a link to something you mentioned. If they still don't respond, leave it. Do not send a third message, do not comment on their posts to get their attention, and do not message from a second account. Silence on a request from someone who doesn't follow you is a clear answer.