Can some stars not emit any energy in the visible spectrum? Stars convert mass into energy. Even converting the slightest mass into energy is immense because the speed of light is so great and $E = mc^2$. This means that stars have to emit large amounts of energy. So can they emit so much energy in a form of electromagnetic radiation that is not visible light? Resulting in a star that cannot be detected using our senses(+ a telescope). If so what problems could this star pose?
Light that is not light


That's meaningless.  All light is electromagnetic radiation.  A finite part of the infinitely large range of the electromagnetic spectrum is visible light.  So you should talk about EM radiation and to discuss the visible spectrum just say visible spectrum.

Stars emit lots of energy at frequencies that are outside the visible range.


  Dark Matter


Dark matter does not mean that it's just not radiating in the visible part of the spectrum.  It means it's not radiating in any part of the EM spectrum much.  If dark matter exists (and that's not certain) it won't interact with most matter much at all.

If dark matter were in star-like objects, it sounds very unlikely that there would not be any EM radiation as a result of the interactions that would result in a star-like object.  We'd detect it if it were there.  So I don't think that's likely.