The domain of a quadratic function is typically all real numbers, since there are no restrictions on the values that \( x \) can take. The function \( y = x^2 - 8x - 20 \) does not have any denominators that could cause division by zero or any square roots that could lead to negative values. Therefore, the domain of this function is:
\[ \text{Domain: } (-\infty, \infty) \]