&gt;&gt; In cosmology, you can only observe, not experiment<p>&gt;Saying that is like saying &quot;we don&#x27;t know evolution is true, because we don&#x27;t see new species evolve every day&quot;. It&#x27;s a naive approach to science.<p>In general it&#x27;s also true of almost all astronomical research, and large whacks of geology. Which is fine because experiments are just observations that scientists are the impetus for. 
Science works by gathering data, experiments just let us produce data <i>to</i> gather at our own speed instead of having to wait around for something to happen and hope we&#x27;re looking in the right direction when it does.
In the case of astronomy instead of relying on experiments to produce more data we mostly build better instruments.