[Tex/LaTex] Vertical space between matrices

Question

I am writing some lecture notes in Econometrics and I want your advice on how to insert some vertical space between two matrices so the result will be easier to follow.

Here is the minimal example:

\documentclass[10pt,a4paper]{article}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}

\begin{document}

\begin{align*}
Var(u) &= E(uu') = \\
&= E
\begin{bmatrix}
u_1 u_1' & u_1 u_2' & \cdots & u_1 u_M' \\
u_2 u_1' & u_2 u_2' & \cdots & u_2 u_M' \\
\vdots & & \ddots & \vdots \\
u_M u_1' & u_M u_2' & \cdots & u_M u_M '
\end{bmatrix}
=
\begin{bmatrix}
\sigma_{11} I & \sigma_{12} I  & \cdots & \sigma_{1M} I \\
\sigma_{12} I & \sigma_{22} I  & \cdots & \sigma_{2M} I \\
\vdots &  & \ddots & \vdots\\
\sigma_{1n} I & \sigma_{2M} I & \cdots & \sigma_{MM} I
\end{bmatrix} \\
&= 
\begin{pmatrix}
\sigma_{11} & \sigma_{12} & \cdots & \sigma_{1M} \\
\sigma_{12} & \sigma_{22} & \cdots & \sigma_{2M} \\
\vdots &  & \ddots & \vdots \\
\sigma_{1M} & \sigma_{2M} & \cdots & \sigma_{MM}
\end{pmatrix} \otimes I_T
\end{align*}

\end{document}

Answer 1

To illustrate @JohnKormylo's comment, you would need to put it in place of the \\ you have already. Changing your MWE (for a gap of 10pt):

\documentclass[10pt,a4paper]{article}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}

\begin{document}

\begin{align*}
Var(u) &= E(uu') = \\
&= E
\begin{bmatrix}
u_1 u_1' & u_1 u_2' & \cdots & u_1 u_M' \\
u_2 u_1' & u_2 u_2' & \cdots & u_2 u_M' \\
\vdots & & \ddots & \vdots \\
u_M u_1' & u_M u_2' & \cdots & u_M u_M '
\end{bmatrix}
=
\begin{bmatrix}
\sigma_{11} I & \sigma_{12} I  & \cdots & \sigma_{1M} I \\
\sigma_{12} I & \sigma_{22} I  & \cdots & \sigma_{2M} I \\
\vdots &  & \ddots & \vdots\\
\sigma_{1n} I & \sigma_{2M} I & \cdots & \sigma_{MM} I
\end{bmatrix} \\[10pt]  %%%%%%%%%%%%%%%%% Change this for different spacing
&= 
\begin{pmatrix}
\sigma_{11} & \sigma_{12} & \cdots & \sigma_{1M} \\
\sigma_{12} & \sigma_{22} & \cdots & \sigma_{2M} \\
\vdots &  & \ddots & \vdots \\
\sigma_{1M} & \sigma_{2M} & \cdots & \sigma_{MM}
\end{pmatrix}_{\otimes I_T}
\end{align*}

\end{document}\documentclass[]{article}

\begin{document}

\title{Title}
\author{Author}
\date{Today}
\maketitle

Content

\end{document}