ggplot alpha levels appear different on fill and border of points (ringing artefact)

Question

I am plotting many points using ggplot with a constant transparency value for all points.

What I find is that the circular points have a more transparent fill than

Answer 1

Update: Tom Wenseleers solution (accepted) is better than the below.

After discussion with @42, the solution is that the PNG default had resolution low enough that at the border between a marker and the image background there was a blending artifact (might not be the right terminology).

Increasing the dpi solves the issue, and adding stroke=0 looks a bit better.

ggsave("plot.png",
  ggplot(data, aes(x,y)) + 
  geom_point(alpha=0.2, color="dodgerblue", fill="dodgerblue", size=4, stroke=0) +
  theme(panel.background = element_rect(fill = 'black', colour = 'black')),
  dpi=1200)