To shift the graph of the function \( f(x) = -2x^2 \) down by 5 units, you need to subtract 5 from the original function.
The new function \( g(x) \) will be:
\[ g(x) = f(x) - 5 = -2x^2 - 5 \]
Therefore, the equation of the function \( g(x) \) that shifts the graph of \( f(x) \) down 5 units is:
\[ g(x) = -2x^2 - 5 \]