Is it possible to run a windows 10 universal app on windows 8.1? I searched a lot, but I didn't find a clear answer on MSDN etc.
A Universal Windows app is a Windows experience that is built upon the Universal Windows Platform (UWP), which was first introduced in Windows 8 as the Windows Runtime. Universal Windows apps are most often distributed via the Windows Store (but can also be side-loaded), and are most often packaged and distributed using the .APPX packaging format.
"The best way to be ready for Windows 10 is to build your Universal Windows app for Windows 8.1 and Windows Phone 8.1 now."- Microsoft
How to Build a Windows 8.1 universal app reference
The answer would be no. For further clarifications, best to post on the forum: https://social.msdn.microsoft.com/Forums/windowsapps/en-US/home?forum=wpdevelop
Windows 10 introduces several new APIs and concepts (such as API contracts) that are not available on Windows 8.1. An app that relies on such new APIs and concepts cannot run on Windows 8.1. Additionally the app-model and a lot of packaging machinery for Windows 10 Universal apps is very different from Windows 8.1. Apps built with such new information cannot be successfully understood by Windows 8.1 for deployment. Also, Store will not make the Windows 10 apps available for download on Windows 8.1