"Karma" is a bit of loaded term - or maybe just a misunderstood one.
When I discuss Karma, especially around my cohort of scientifically minded, anti-superstitious compatriots, there's a fair bit of eye-rolling and shrugging. I think this is primarily because of the Westernized definition of Karma: a singular deliberate action of a certain nature will be repaid, in time, against the actor, in similar kind. Karma has become some sort of “golden-rule” or euphemism for “what goes around, comes around”. This sort of linearity or direct cause-and-effect is something we hesitate to apply to emotional things like the morality of a specific deed.
The original (Hindu) definition is more along the lines of the following: it is not punishment or retribution but simply an extended expression or consequence of natural acts. This broader definition of the way mindfulness or positivity in one's actions will pervade one's surroundings (including that which (seemingly) acts from outside onto the perceiver), is the idea of Karma that makes sense to me.
If I say something nasty to someone, I have no reason to believe that the universe (outside of me) is going to lash out and do something nasty to me in return. That’s the linear causality that my Western friends roll their eyes at… and I suppose I do too.
My preface is this: you are the ultimate perceiver and interpreter of your own universe, and the barrier between you and the universe is a bit of a trick (where do your eyes end and the light begin?). So, as you produce these nasties with your mind, you are in fact manifesting or channeling some nastiness in the universe and acting to continue its propagation, as a host organism for the sour pathogen. This taints your actions as well as your sensations (perceived as the actions of the universe).
Karma in this sense exists - with a little explanation of my thinking, it seems pretty clear that that which you generate, does indeed change the way the universe treats you.