If there's something in the air that prevents us from seeing more than 25 miles, what is it? We would have been able to identify it by now.
We have. They're called "atoms."
Why exactly 25miles tom?? Why tom?
The air density at sea level, may be calculated as: D = (101325) / (287.05 * (15 + 273.15)) = 1.2250 kg/m3
Over a distance of 25 miles it becomes simply too dense to see through.
Dense is the wrong word. You make it sound as if there's a constant change in the density of the air between where you're standing and a circle 25 miles away around you, culminating in a wall.
Air is not transparent. It is translucent. It absorbs, refracts, and diffuses light (for examples of this, look at the sky). If you stand more than a certain distance away, enough of the light bouncing off of an object in your direction will be absorbed, refracted, or diffused that the object is not clearly visible. This effect will get more pronounced over longer distances, and outside of 25 miles, you can basically see squat.