I already tried to use Detexify but the closest suggestion was \mathfrak{Y}. I'm sure that it is not the case.
Does anybody have idea how to produce the letter used for the space cited on the image?

You can as well use the yfonts package, following @egreg's answer here.
\documentclass{article}
\usepackage{yfonts}
\begin{document}
\textfrak{Y}
\end{document}

\mathfrak{Y}, for instance fromesstixfonts (seemathalfa). – Manuel Feb 03 '15 at 23:46