Gamma-ray bursts (GRBs) are short flashes of photons thought to originate from collapsing stars. These strong pulses have energies in the X-ray to gamma-ray ranges and time scales ranging from a few milliseconds to around 100 seconds. The separation between "short" and "long" bursts is around two seconds.
We cannot see one of the universe’s primary constituents: dark matter. The reason is simple: it's dark. But we can infer where it is located from observations of distant galaxies because of a key property of light, namely that it does not always travel in straight lines.