Google has added a new 'Archive' feature to Photos that should be showing up for users if they're on the latest version of the app, though it's also worth mentioning that it might not be showing for everyone just yet, so keep an eye out for it if you happen to be one of the users who doesn't yet have access to it. As one may expect Archive works as it sounds, it lets you take pictures which are showing up in the main page of the app and move them out of view. They'll still be stored on your device so you won't be deleting them, you'll simply be moving them to a special location so they're no longer visible in the main section.
To access the archived photos you can tap the hamburger menu button to bring up the list of different app sections, and the Archive page option should be towards the top. There will be a "new" tag in blue lettering next to it so it'll be hard to miss, and if you have the feature available an alert should pop up the next time you open the Photos app. Inside the Archive menu you'll see any photos which you've archived previously unless you haven't actually archived any yet. You can add photos to the Archive menu from within it by tapping the add icon in the top right corner, but you can also add photos to the Archive menu individually from the overflow menu options for each photo if you're viewing them from the main page. Naturally, you can also multi-select photos, hit the overflow menu from the main page, and the archive option will appear here as well.
Once you archive a photo Google notifies you that you can still see the picture in your albums, if you have any of the archived photos in albums to begin with, and you can still find it by searching for it. You also get an option to immediately go to the Archive page from the pop up that's displayed after archiving your first photo. Just as easy as it is to send a photo to the archive, you can unarchive photos and send them back to the main page for viewing if you wish. While this may not be used by everyone, the point of the feature is to allow users to clean up their main photos page and leave it a little bit more organized.