cells made entirely out of metal.
There's really no reason why metallic life forms can't exist, it's just that we haven't met any and nobody's sure how it would work. To prove that it's at least physically possible, a team from the University of Glasgow has created some cell-like bubbles call iCHELLs out of metallic elements like tungsten bonded with oxygen and phosphorus. These bubbles can self-assemble, and they exhibit many of the same properties that allow biological cells to do what they do, including an internal structure and a selectively porous outer membrane that can let other molecules pass through. It may even be possible to set the metallic cells up to perform photosynthesis.
The tricky bit at this point is to figure out how to imbue the metallic cells with something like DNA to allow them to self-replicate and evolve, but this may in fact be possible: the bubbles can use each other as templates to create more bubbles, and experiments suggest that they may even be able to alter their own chemistry to adapt to different environments. If all of this works out to the point where we can at least say that yes, it's definitely possible for metal life forms to have evolved, it would have massive implications for our search for extraterrestrial life.