Moments in Photos are the new Events, i.e. groupings of photos sorted by date taken.
When the iPhoto Library was first migrated to Photos there is a folder created in the sidebar titled iPhoto Events and all migrated iPhoto Events (which are now Moments) are represented by an album in that folder. To open the sidebar if it's not already open use the Option+Command+S key combination.
NOTE: It's been reported by several users that if the Event albums are moved out of the iPhoto Library folder in the sidebar they may disappear. It's not widespread but several users have reported that issue. Therefore, if you want to assure that you keep those Event albums don't move them outside the iPhoto Events folder.
There's a way to simulate events in Photos.
When new photos are imported into the Photos library go to the Last Import smart album, select all the photos and use the File ➙ New Album menu option or use the key combination Command+N. Name it as desired. It will appear just above the iPhoto Events folder where you can drag it into the iPhoto Events folder
When you click on the iPhoto Events folder you'll get a simulated iPhoto Events window.
The downside to the simulation is that the Albums/Events can only be sorted automatically by Title. But they can also be sorted manually, either in the sidebar or in the folder's window at the right. This will change with the release of Photos 2 with El Capitan this fall. That will allow the sorting by date or title, ascending or descending.
Tell Apple what missing features you'd like restored or new features added in Photos via Apple-Photos Feedback.
iPhoto was not deleted from your hard drive, only removed from the Dock. If you have iPhoto 9.6.1 you can still use your iPhoto library.
If you don't have that version but some earlier version of 9.x obtained from Apple via Software Update try following Terence Devlin's instructions in this post to obtain iPhoto 9.6.1 from the App Store:
Terence Devlin
Apr 14, 2015 11:21 AM
Re: Is Iphoto gone ? i want it back!
in response to Johannes666
Recommended
Go to the App Store and check out the Purchases List. If iPhoto is there then it will be v9.6.1
If it is there, then drag your existing iPhoto app (not the library, just the app) to the trash.
Install the App from the App Store.
Sometimes iPhoto is not visible on the Purchases List. it may be hidden. See this article for details on how to unhide it.
http://support.apple.com/kb/HT4928
One question often asked: Will I lose my Photos if I reinstall?
iPhoto the application and the iPhoto Library are two different parts of the iPhoto programme. So, reinstalling the app should not affect the Library. BUT you should always have a back up before doing this kind of work. Always.