Athiest means to not believe in God, thus your statement makes no sense. If it were necessary to have something other than Athiesm there wouldn't be any athiests. If the replacement for raw athiesm is something other than God, it's still athiesm if you don't believe in a God.

We do understand what gravity is, it's a property of mass which can be measured and accurately predicted. To most, this is enough, to others it gets deeper into space/time distortions and possibly the existence of gravitons. Einstein had a lot to say on the subject .

Anger also is a comprehensable phenomenon, an evolutionary instinct made intricate by the convoluted modern hemispheres interfering with our basic survival traits. You can see anger in many forms but like all emotions it can be many 'shades' of emotion subsumed under an umberella term....A term decided upon by social convention and categorized by social norms. We percieve sadness and happiness as being clear states of being like being hot or cold but it is the magic of language that simplifies the complex neurological, psychological and physiological into clear cut signs and signifieds.