The only values of x where the domain is not defined is where x^2 - 4 = 0
So
x^2 - 4 = 0
(x + 2)(x - 2) = 0
Setting each factor to 0 and solving for x gives us that x = -2 and x = 2
Look at the graph here : https://www.desmos.com/calculator/ph4j7rnvab
The domain exists on (-infinity, -2) U (-2, 2) U (2, infinity)