I read the article, but the headline's question makes me want to type. We are. It's gravity, the inhospitable nature of space for humans, and the vast distances involved in the universe. Of those, gravity is the most interesting. Gravity is the weakest natural force in classical physics. It's much weaker than things like magnetism. It's also very poorly understood. We don't have a complete understanding of gravity. We can observe it. We can quantify it. We can use models to accurately predict it - and have been able to since the arrival of Newtonian physics. (Newtonian physics are more than adequate to put a man on the moon, or plot a path for a space probe.) But, we have no idea what causes it. We know it's a function of mass, but not why. We have no clue why it does what it does. There's even some strange (and much too complicated for me to fully understand) discussion about the speed of gravity and how far the force can travel. In short, we don't actually know.