The current issue of New Scientist has a summary of 21st Century Cosmology: "Dark Matter". The article itself is most accessible; Different fields of work in this complex science are related.
Our species is awaiting the outcomes of several experiments designed to produce or detect a substance that, by definition, cannot be produced or detected. Our existence (universe) depends, it turns on, the existance of this First Substance.
Our own solar system is located about 28,000 light years from galactic center of the Milky Way. It orbits the centre of the Galaxy at a speed of 274 kilometres per second. So how long does one orbit take?
Hint #1: You're not going to need a scientific calculator, just Excel-we'll do this "sailor style" (memo to Captn. Kirk-we're not computing for time warp.) Use this first. Remember the formula for the circumference of a circle? We're going to divide this by distance/year: Speed times the seconds/year. The scientific notation factor for the number of seconds in a year is remarkably similar to pi, so these will cancel.
Modern Anthropology holds that "civilization" (agriculture) began about 12,000 years ago. Exactly how this could be detected from any point above the Earth's atmosphere totally baffles me - Unless a 2001: Space scenario can be proved plausible. Modern Anthropology is totally baffled by the time frame for the appearance of H. Sapiens (us) so we'll ignore this.
Hint #2: Taking the fraction 2/274 as a factor 7.3x 10-3, multiply
Minus aliens all we have is the invention of radio: Marconi sent and received his first radio signal in Italy in 1895. By 1899 he flashed the first wireless signal across the English Channel and two years later received the letter "S", telegraphed from England to Newfoundland. This was the first successful transatlantic radiotelegraph message in 1902; That's 107 years ago or .892% of the span of civilization.