Expansion is in vacuum, so work done by expanding gas is $\Delta W=0$.

Container is insulated, so heat exchanged with surroundings is $\Delta Q=0$.

Now, from first law of thermodynamics, we have

$\Rightarrow \Delta Q=\Delta U+\Delta W$

$\Rightarrow \Delta U=0$

$\Rightarrow U_i=U_f$

Hence, initial and final internal energies of gas are equal.

As, the gas is not ideal expansion results in reduction of pressure, so intermolecular potential energy increases. This results in reduction of kinetic energy, so temperature reduces.