6. The solution set of the inequality
$$
x^{6}+3 x^{5}+5 x^{4}+3 x^{3}-2 x^{2}-1<0
$$

is $\qquad$ .
6. $\left(\frac{-1-\sqrt{5}}{2}, \frac{-1+\sqrt{5}}{2}\right)$.

Define the increasing function $f(x)=x^{3}+2 x$.
Solve the problem in segments below.
(1) Clearly, $x=0$ is a solution to the inequality.
(2) When $x \in(0,+\infty)$, the original inequality becomes
$$
\begin{array}{l}
x^{3}+3 x^{2}+5 x+3-\frac{2}{x}-\frac{1}{x^{3}}f\left(\frac{1}{x}\right) \text {. }
\end{array}
$$

Similarly, $\frac{-1-\sqrt{5}}{2}<x<0$.
In summary, the solution set of the original inequality is
$$
\left(\frac{-1-\sqrt{5}}{2}, \frac{-1+\sqrt{5}}{2}\right)
$$