There is plenty of theory: the Compromised Defense, Main Line with its main sub-variations, the Paulsen and the Potter. Then there are the variations, Leonhardt, Sokolosky, Sanders-Alapin and the Alapin-Steinitz; the attacks, Tartakower, Levenfish, Richardson and the Waller; and the Lasker Defense, to name a few.
The Evans was once popular, but then along came Wilhelm Steinitz who advocated a torturous defense against it. He was followed by Emanuel Lasker who employed a defensive plan that pretty much relegated the Evans Gambit to obscurity.
The main problem for white was that Lasker's Defense was so sound that it was considered a problem and white avoided it by playing 6.d4 and meeting 6...d6 with 7.Qb3, but even that offered little more than equal chances.
Though black has never refuted the Evans, modern play has uncovered lines that pretty much guarantee him easy equality. Of course white has to be prepared to meet the Two Knights Defense which involves delving into even more complicated territory where a lot of theory has to be digested. Still, the Evans can be fun to play on occasion. I don't recommend it in modern correspondence play though. I tried it a couple of years ago and even using Stockfish and examining a couple of dozen games looking for promising lines, the game soon fizzled out to a draw.
In the 1980s Jan Timman and John Nunn used it on rare occasions, but then in 1995 when Kasparov defeated Anand in the Tahl Memorial in Riga in 1995 the gambit experienced a revival. But, as you might expect, its popularity again faded and it's rarely seen these days.
I was going to analyze the game Shirov vs. Timman from Biel 1995 and in the process of doing some research on the opening I discovered that Shirov himself analyzed the game on Vimeo HERE. Naturally his analysis is better than anything I could do, but because he didn't give much analysis on the opening in the video, I will present the game with opening comments, then you can set across from Shirov and let him explain his play to you. Chessdotcom also has some interesting article on the Evans HERE.