My wife and I just recently bought a house. The front door threshold "rolls" as you walk over it because there is a gap of about 1/4 to 1/2 inch between the threshold and the concrete (on the outside of the home) and probably goes approximately 1/2 to 1inch under the threshold. This is an aluminum threshold. What can I use to fill this gap?

I was told that concrete should have been added during door installation to fill this gap. Is there something else I can use to fill this gap? such as rubber, foam etc... I was hoping for a quick fix because I do not want to remove the double doors for a 1/2 inch gap. Any help would be appreciated. Thanks