How do we know that there aren't more elements in the Universe other than those on the periodic table?

Honestly, it'll take a long time to find out if there are other elements in the universe(which there actually are). When you think about it, the universe is a pretty big place and it is no surprise that there are at least a million different elements that we haven't found. Heck there might actually be a cure for death out there, but what I'm trying to say is we don't really know YET. And the only real answer that I can give you right now is time. Time will answer it for us. Soon...