SOURCE

/**
 * Just a very basic vector 
 */
function Vector(x, y) {
  this.x = x ? x : 0;
  this.y = y ? y : 0;
  
  this.add = function(x, y) {
    if (x instanceof Vector) {
      this.x += x.x;
      this.y += x.y;
      
      return this;
    } 
    
    this.x += x;
    this.y += y;
    
    return this;
  }
  
  this.subtract = function(x, y) {
    if (x instanceof Vector) {
      this.x -= x.x;
      this.y -= x.y;
      
      return this;
    } 
    
    this.x -= x;
    this.y -= y;
    
    return this;
  }
  
  this.multiply = function(x, y) {
    if (x instanceof Vector) {
      this.x *= x.x;
      this.y *= x.y;
      
      return this;
    } 
    
    this.x *= x;
    this.y *= y;
    
    return this;
  }
  
  this.divide = function(x, y) {
    if (x instanceof Vector) {
      this.x /= x.x;
      this.y /= x.y;
      
      return this;
    } 
    
    this.x /= x;
    this.y /= y;
    
    return this;
  }
  
  this.normalize = function() {
    return this.magnitude() === 0 ? this : this.divide(this.magnitude(), this.magnitude());
  }
  
  this.magnitude = function() {
    return Math.sqrt(this.magnitudeSquared());
  }
  
  this.setMagnitude = function(val) {
    return this.normalize().multiply(val, val);
  }
  
  this.magnitudeSquared = function() {
    var x = this.x, y = this.y;
    
    return (x * x + y * y);
  }
}

function Attractor(x, y) {
  this.position = new Vector(x, y);
  this.G = 10;
}

function Particle(x, y) {
  this.position = new Vector(x, y);
  this.velocity = new Vector(randMinus(0.2), randMinus(0.2));
  this.acceleration = new Vector();
  this.size = 1;
  
  this.attract = function(attractor) {
    var force = new Vector(attractor.position.x, attractor.position.y);
    force = force.subtract(this.position);
    
    var dsquared = force.magnitudeSquared();
    dsquared = Math.max(Math.min(dsquared, 500), 25);
    
    var strength = attractor.G / dsquared;
    force.setMagnitude(strength);
    
    this.acceleration = force;
  }
  
  this.update = function() {
    this.position.add(this.velocity);
    this.velocity.add(this.acceleration);
  }
  
  this.draw = function(context) {
    context.fillRect(this.position.x, this.position.y, this.size, this.size);
  }
}

function randMinus(num) {
  if (Math.random() > 0.5) {
    return -num;
  }
  
  return num;
}

(function(){
  var canvas = document.getElementById('canvas'),
      ctx = canvas.getContext('2d'),
      w = canvas.width = window.innerWidth,
      h = canvas.height = window.innerHeight,
      density = 1,
      attractors = [],
      particles = [];
  
  function setup()
  {
    window.addEventListener('resize', resize);
    
    density = window.devicePixelRatio != undefined ? window.devicePixelRatio : 1.0; 
    
    canvas.width = w * density;
		canvas.height = h * density;
		
		ctx.scale(density,density); 
    
    var size = Math.min(w, h) / 5;
      
    for (var i = 0; i < 750; i++) {
      var x = Math.sin(i) * size;
      var y = Math.cos(i) * size;
      
      particles.push(new Particle((w / 3) + x, (h/2) + y));
    }
    
    var middle = new Attractor(w/2, h/2);
    middle.G = 5;
    
    //var left = new Attractor(w/3, h/2);
    //left.G = 5;
    
    //var right = new Attractor((w/3)*2, h/2);
    //right.G = 10;
    
    attractors.push(middle);
    
    //attractors.push(right);
    //attractors.push(left);
    
    draw();
  }
  
  function draw()
  {
    ctx.fillStyle = 'rgba(12, 50, 80, 0.05)';
    
    for (var i = 0; i < particles.length; i++) {
      var particle = particles[i];
      
      for (var n = 0; n < attractors.length; n++) {
        particle.attract(attractors[n]);
      }
      
      particle.update();
      particle.draw(ctx);
    }
    
    window.requestAnimationFrame(draw);
  }
  
  function resize()
  {
    w = canvas.width = window.innerWidth;
    h = canvas.height = window.innerHeight;
    
    canvas.width = w * density;
		canvas.height = h * density;
    
    ctx.scale(density, density);
  }
  
  setup();
})();

<canvas id="canvas"></canvas>

body
{
  background: #efefef;
  overflow: hidden;
}

canvas
{
  width: 100%;
  height: 100%;
  position: absolute;
  left: 0;
  top: 0;
}