monospaced fonts not respecting spaces?

\texttt just sets the typeface. It doesn't change how TeX sees spaces. If you want that, then you need to use some kind of verbatim macro or environment. Or fiddle with the catcode of space (NOT RECOMMENDED!)

You can use the package listings or the environment verbatim. An example you will see in the following MWE. Could it be that a table would better fit your issue?

%http://tex.stackexchange.com/questions/70007/monospaced-fonts-not-respecting-spaces
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{listings}
\begin{document}
\par\texttt{this is monospaced text    Where  is  the  spaces  ?    !!!}

\begin{verbatim}
this is monospaced text    Where  is  the  spaces  ?    !!!
\end{verbatim}

\begin{lstlisting}[basicstyle=\ttfamily]
this is monospaced text    Where  is  the  spaces  ?    !!!
\end{lstlisting}

\begin{lstlisting}[basicstyle=\ttfamily,showspaces=true]
this is monospaced text    Where  is  the  spaces  ?    !!!
\end{lstlisting}

\end{document}​

Another approach for short verbatim text is to use the \Verb command from the fancyvrb package; I added the obeytabs option and used tabsize to recover the alignment inside the \matrix (the characters are spaced using tabs):

Apparently, when I copy the code from my editor to this site the tabs get lost. You need to use A(tab)B(tab)C(tab)D and Ac(tab)Ad(tab)Be(tab)Cf in the code below, where (tab) stands for a tab in your editor:

\documentclass{article}
\usepackage{fancyvrb}
\usepackage{tikz}
\usetikzlibrary{matrix}

\fvset{obeytabs}

\begin{document}

\begin{tikzpicture}%
\matrix (m) [matrix of nodes, every node/.style={font=\ttfamily},%
    row 1/.style={text width=7em, align=left},
    row 2/.style={text width=7em, align=left}]
    {
\Verb[tabsize=2]+A  B   C   D+\\
\Verb[tabsize=1]+Ac Ad  Be  Cf+\\
    };
\end{tikzpicture}

\end{document}​

Using the \lstinline command from the listings package, there's no need to use tabs; simple spaces will behave as expected:

\documentclass{article}
\usepackage{listings}
\usepackage{tikz}
\usetikzlibrary{matrix}

\begin{document}

\begin{tikzpicture}%
\matrix (m) [matrix of nodes, every node/.style={font=\ttfamily},%
    row 1/.style={text width=7em, align=left},
    row 2/.style={text width=7em, align=left}]
    {
\lstinline+A  B  C  D+\\
\lstinline+Ac Ad Be Cf+\\
    };
\end{tikzpicture}

\end{document}