It is extremely difficult for modifications of gravity to simultaneously fit the cosmic microwave background, big-bang nucleosynthesis, galactic rotation curves, and the Bullet Cluster. The safe bet these days is on some sort of non-baryonic dark particle (which is why so many people are looking for them). Most other loopholes are closed or in the process of being closed.
The coming decade or two should have something significant to say about dark matter. We will either discover its nature or severely constrain its properties through a plethora of varied experiments and searches.
It's a bit like exploding an apple and measuring all the bits that fly of and missing some bits. Whilst we do not know what the apple looked like initially, we are from our measurements, trying to reconstruct that apple.
How convinced are we that their is dark matter and not a case of how we have measured things so far. Hence, differences in results based upon how we measure are evident, whilst many are close to the same, they just don't exactly match up - hence - dark matter theory.
How likely is it that dark matter as we know it - does exist?
Of course you are risking that he eventually proves your hover board is impossible.