Here is a quick bar plot from a recent consulting engagement on ETFs managed by US Funds.

The ETF universe of approximately 2,300 funds are dominated by Equity funds, followed by Bond funds.

The remaining asset classes have a fair bit to catchup and can provide good diversification for ETF investors.

Code to generate the above plot is:

library(tidyverse)
library(lubridate)
library(ggthemr)
library(cowplot)


dt_asset_class_summary <- dt_consolidated[,.(assets=sum(assets_f)),by=asset_class][order(assets, decreasing = TRUE)]

#set theme
ggthemr("light", layout = "plain", text_size = 12)

# credit: https://github.com/cttobin/ggthemr#a-note-about-theme-setting
ggthemr_reset()
dust_theme <- ggthemr('dust', set_theme = FALSE)

etf_aum_bar_plot <- ggplot(dt_asset_class_summary) +
  geom_bar(stat = "identity", aes(x=asset_class, y = assets,
               group=as.character(asset_class), 
               fill=as.character(asset_class))) +
  xlab("Type of ETF") +
  ylab("Asset Volume (MM$)") +
  labs(title="ETF Distribution by Asset Class",
       subtitle="Distribution as of 12-Feb-2020") +
  scale_fill_discrete(name = "asset_class")

etf_aum_bar_plot + dust_theme$theme