Wiktionary
untestable
a. That cannot be tested; unverifiable
Usage examples of "untestable".
What such critics for their part fail to realize is just how difficult scientific research actually is, how complex the testing of any even seemingly trivial hypothesis or hunch may be, and how many paradoxes and seeming mysteries we confront every day in our research which to us are at least as challenging as, but theoretically more relevant than, fretting about probably untestable phenomena like ESP.