how can I get the thicker x symbol like the following picture?

Question

I try the \boldsymbol{\times}, but it doesn't look right.

Answer 1

It is bit heavy, but highly customizable.... using tikz:

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{shapes.misc}
\newcommand{\mytimes}{ \tikz[baseline=-.55ex] \node [inner sep=0pt,cross out,draw,line width=1pt,minimum size=1ex] (a) {};}
\begin{document}
  \[\begin{bmatrix}
    \times    & \mytimes \\
    \mytimes  & \times
  \end{bmatrix}
  \]
\end{document}

Here is one with bm:

\documentclass{article}
\usepackage{amsmath}
\usepackage{bm}
\begin{document}
  \[\begin{bmatrix}
    \times    & \bm{\times} \\
    \bm{\times}  & \times
  \end{bmatrix}
  \]
\end{document}