根据题意，有 $x^3+y^3=0$，于是$$(x+y)\left(x^2-xy+y^2\right)=0.$$若 $x+y=0$，则$$\left(\dfrac{x}{x-y}\right)^{2000}=\left(\dfrac{y}{y-x}\right)^{2000}=\dfrac{1}{2^{2000}},$$于是原式的值为 $\dfrac{1}{2^{1999}}$．
若 $x^2-xy+y^2=0$，设 $z=\dfrac{x}{y}$，则$$z^2-z+1=0,$$进而 $z=\cos\dfrac{\pi}3+{\rm i}\sin\dfrac{\pi}3$ 或 $\overline{z}=\cos\dfrac{\pi}3+{\rm i}\sin\dfrac{\pi}3$，进而\[\begin{split} \left(\dfrac{x}{x-y}\right)^{2000}+\left(\dfrac{y}{y-x}\right)^{2000}&=\left(\dfrac{z}{z-1}\right)^{2000}+\left(\dfrac{1}{1-z}\right)^{2000}\\
&=\left(\dfrac{z^2}{z^2-z}\right)^{2000}+\left(\dfrac{1}{-z^2}\right)^{2000}\\
&=z^{4000}+{\overline z}^{4000}\\
&=2\cos\dfrac{4000\pi}3\\
&=-1.\end{split}\]综上所述，所求代数式的值为 $\dfrac{1}{2^{1999}}$ 或 $-1$．