This substance became known as Dark Matter. Since then numerous experiments have been devised to test this theory. Vera Rubin showed that galaxies rotate as fast at the outer rim, or faster, as they do in the inner. If they are less dense at the outer edges, as the luminosity indicates, then they should move slower. Jeremiah Ostriker and James Peebles showed that without extra matter, galaxies would develope gravitational anomalies that would cause them to collapse into other forms than the spiral we usually see.
Nobody knows, but there are many theories.
So far neither Magnetic Monopoles, nor Gravitinos have been found. Arcane knowledge povides some answers assuming we ask the questions. Being the opposite side of Isis we have to assume she plays a part in Universe. The answer to both of these lies in the Dark Matter. I found this very intriquing at the time, and later found the connection in Isis's dark twin, Nepthys.