I am not a physicist, but my intuition tells me we are missing an element or multiple elements from the periodic table that defy gravity.
I think this because there are many elements, each with varying physical and chemical properties, that make them unique. Each element differs from the others in some way, shape or form.
Long story short, is that I believe there is an element or particle that opposes gravity. It only seems logical that this is a very real possibility, given the wide range of properties that known elements display.
The problem is that we would have a hard time finding this element on earth because when released it would fly up into space. This element or particle would also probably oppose other particles, given the fact that gravity, to my understanding, is the attraction of any particle to another particle. Mass of an object dictates its gravitational forces. Given these criteria, a particle that opposes gravity, would also oppose every little particle and possibly even itself or particles similar to it. This would mean that the said particle would only be found by itself, in a 3 dimentional void between massive objects or in areas of outer space where gravitational forces are not present.
If this type of particle could be found or captured, the possibilities of its uses are endless. Harnessing this type of energy could make space travel easy and would make flying a "breeze". This would also be an element that could create energy without using energy.