Because that's literally the definition of dark matter. It's stuff we can't see that contribute to gravity. But, we can use this contribution to describe quantities of it. And no they shouldn't be, gamma ray bursts are highly sensationalized, even though they are very dangerous, there's just nothing immediately around us that poses that danger. Pulsars are actually a very important tool that moving forward will become vital for understanding dark matter and gravitational waves, as they're basically like standing clocks on the middle of space.