is there a restaurant in the museeum

No. But adjacent to

There is a small cafe in the Shop at the Denver Art Museum on level one. In addition, there is a Mad Greens across Martin Plaza and other restaurants within a block or two. When the North Building renovation is complete, there will be a restaurant and cafe there.

There is a Mad Greens almost directly opposite the Museum entrance, and as mentioned, a small Cafe inside.

I did not see one, but there are some in very near walking distances.

There's a cafe