I think my husband is gay. What should I do?

I'm sorry to hear this. What does he say is an excuse in terms of having that particular app? That particular app is known for men wanting to meet other men. I have no idea what his plausible explanation would be. If he doesn't accept it, then I would definitely involve a third party, whether it be your local scholar, your local imam, your local Sharia council, to try and get to the bottom of this. That's definitely what I would suggest. Also just make sure that you are, from a health perspective, if he has been meeting other people, that you are healthy. If he's not willing to engage with you, involve a third party he respects, whether it be a local scholar, local imam, Sharia council, to try and get to the bottom of this particular issue.