Horizontal alignment under a character (word)

by use of the tabular:

\documentclass{article}

\begin{document}

\begin{itemize}
\item   In outer space,\\
        the change in pressure is dominated 
            \begin{tabular}[t]{@{} l}
            by unicorns at low temperature;\\
            by gnomes at moderate temperature; and\\
            by dwarves at high temperature.
            \end{tabular}

\item   In outer space,\\
        the change in pressure is dominated
            \begin{tabular}[t]{@{} l}
            by unicorns at low temperature;\\
            by gnomes at moderate temperature; and\\
            by dwarves at high temperature.
            \end{tabular}
\end{itemize}

\end{document}

You could use \phantom for example as follows:

\documentclass{article}
\usepackage{geometry}
\newcommand{\myplaceholder}{\phantom{the change in pressure is dominated }}

\begin{document}

\begin{itemize}
\item{In outer space,\\
the change in pressure is dominated by unicorns at low temperature;\\
\myplaceholder by gnomes at moderate temperature; and\\
\myplaceholder by dwarves at high temperature.}
\item{Underwater,\\
the change in pressure is dominated by lemmings at low temperature;\\
\myplaceholder by elves at moderate temperature; and\\
\myplaceholder by trolls at high temperature.}
\end{itemize}

\end{document}

---

Here is an alternative version using \widthof from the calc package in combination with \hspace:

\documentclass{article}
\usepackage{geometry}

\usepackage{calc}
\newlength{\mylength}
\setlength{\mylength}{\widthof{the change in pressure is dominated}}

\begin{document}

\begin{itemize}
\item{In outer space,\\
the change in pressure is dominated by unicorns at low temperature;

\hspace{\mylength} by gnomes at moderate temperature; and

\hspace{\mylength} by dwarves at high temperature.}
\item{Underwater,\\
the change in pressure is dominated by lemmings at low temperature;

\hspace{\mylength} by elves at moderate temperature; and

\hspace{\mylength} by trolls at high temperature.}
\end{itemize}

\end{document}

With a \Longunderstack.

\documentclass{article}
\usepackage{stackengine}
\setstackEOL{\\}
\begin{document}

\begin{itemize}
\item In outer space,\\
the change in pressure is dominated \Longunderstack[l]{%
by unicorns at low temperature;\\
by gnomes at moderate temperature; and\\
by dwarves at high temperature.}
\item Underwater,\\
the change in pressure is dominated \Longunderstack[l]{%
by lemmings at low temperature;\\
by elves at moderate temperature; and\\
by trolls at high temperature.}
\end{itemize}

\end{document}